Jessica Mitford sailed in calm and stately fashion across the angry seas of Wisconsin Avenue-against the light, against four lanes of vengeful-looking motorists and against what prudent souls would call common sense.

But then Decca, as she is called by her friends, has rarely cast her lot with prudence. "The truth is," she said, after waiting some minutes for her less courageous companions, "I was desperately afraid of traffic until my son showed me how to think about it in Boston once. He would point at the cars one by one and say, 'Look, think of that one as one of Franco's tanks, and think of that one over there as a subpoena-server from HUAC,' and before I knew it, we were across the square. And that seemed to do the trick."

As evidenced by her traffice patterns, Jessica Mitford does not mind breasting the waves of the established order of things. She is a veteran mocker of what she has seen as evil and absurd in this world. She rejected her place in the English nobility at an early age, served in and quit the American Communist Party, wrote a best seller on her first foray into journalism, and delighted in the howls of praise and protest she has evoked. Att in all, she says, it has made for "a very interesting life."

Now Jessica Mitford is rarely given to such unadorned understatement. "Interesting" hardly goes the distance in describing a life in a family that has made one of the more stunning contributions to the lore of English upper-class eccentricity.

Interesting does not explain her flight to Spain in 1937 to fight the fascists, or her marriage to her second cousin, Winston Churchill's nephew, or the beginning of a writing career at the age of 40 which resulted in her bestselling investigation of the funeral industry, "The American Way of Death."

It is as if a troupe of jugglers arranged her life for her, tossing all the elements and anecdotes into the air to assume some sort of pattern as they fell back to earth.

At 61, Jessica Mitford has assumed her own peculiar niche in the mottled wall of American letters, a lone Englishwoman who helped to reinvigorate the American craft of muckraking, holding up the mirror to some of our stranger customs.

Americans take their sibyis where they find them, but like most of her English compatriots who have come to gape in wonder at our way of doing things and then tell us what they think, Motford is taken more seriously at times than our own home-grown critics.

Her latest book is called "Poison Penmanship: The Gentle Art of Muckraking." It is a collection of her magazine pieces, and includes a running commentary on how she raked the muck for each individual article, as well as a potpourri of anecdotes, mistakes, observations and advice. It includes her all-time favorite piece, a dissection of the Famous Writers' School, in which she takes wicked delight in strangling Bennett Cerf in his own rationlizations.

She is, as Carl Bernstein says in the book's afterword, "a lady who knew something about collecting injustices and setting them straight." Farve and Muv

She has keen eyes that can pierce like a falcon's, and a pleasant amount of plumpness that seems quite content to stay where it is, thank you very much. She talks in italics and sentences that parse, and sits up very straight with her hands folded in perfect composure and quotes her nanny.

"Nanny would always say, 'Don't forget you're the least important person in the room,' which is a salutary thing, when you come to think about it."

All in all, she looks like the archetypical English gentlewoman, a bit dotty perhaps, but quite content to keep to her roses and her prize-winning pugs. Precisely the sort of activities that do occupy her first cousin and are mild occupations indeed compared to the immediate members of the Mitford clan.

First there were the parents of Jessica and her five sisters, Lord and Lday Redesdale. "One of Nature's fascists," was the way two of the Mitford women described farve (father), finding this much to his credit, according to Jessica in "A Fine Old Conflict," the second part of her autobiography. On the other hand, most of muv's young life, until her dubut, was spent on the ocean, which might have accounted for what one obituary called "her imperturbable serenity." She was so serene in fact, according to sister Nancy, that "on one occasion Unity rushed into the drawing room where she was at her writing-table, saying 'Muv, Muv, Decca is standing on the roof-she says she's going to commit suicide!' 'Oh, poor duck,' said my mother, 'I hope she won't do anything so terrible,' and went on writing." Beyond the Pale'

And then there were the sisters. Nancy, glittering among the communion of the chic and the intellectual; writing well-received novels; a cohort, for a time, of Evelyn Waugh; a card-carrying member of the Bright Young People of the post-World War I generation.

Diana, wife of Sir Oswald Mosley, pre-World War II leader of the British fascists-Hitler, Hermann Goering and Frau Goebbels were among their wedding guests.

Unity, Hitler's companion, his "Valkyrie," who was found in Paris with a bullett would in her head, only to linger on in half-life for many years.

Deborah, who always said she would grow up to be a duchess, and with admirable dispatch did exactly that. She is now the Duchess of Devonshire.

Jessica Mitford is a bit bored by all the talk of how extraordinarily eccentric her family appears to those who travel in more furrowed paths. In fact, it got a little embarrassing when there were no less than four books by and about the Mitford girls hitting the stands at the same time.

Her favorite book on the subject was a spoof put out by Private Eye, an English humor magazine, entitled "I Doreen-The Unknown Mitford sister." Twiggy posed on cover as Doreen and the piece contained revelations of the time Doreen poured strychnine into nanny's tea.

"I don't think of ourselves as eccentric," said Mitford. "I suppose each family thinks of itself as unique."

She does not talk to Diana, who is living in France now. She is "really beyond the pale." Jessica is, however, quite fond of Debo, as she calls the Duchess, and of her first cousin with the pugs.

"We start off with my saying, 'Darling, your pugs are too wonderful,' and of course I loathe them, and she says, 'Darling, your books are too wonderful,' and we go on from there. We don't actually have that much in common, but it doesn't matter."

From such a background it would seem that the extraordinary would hang its hat in her doorway as a matter of course. But her friends say and Mitford agrees that she has lived her life "almost as if by accident."

She came to America in 1939 with John Dos Passos' "USA' tucked under an arm "because none of our friends had been there and we wanted to have a look at it." She was then married to Esmond Romilly. They wanted to see the country whose way of life they knew mainly through movies like "Petrified Forest."

"Our impression of the country was that there would be gangster bullets whizzing while couples made love under the tables," Jessica Mitford recalled. And smiled. "Actually, it turned out to be perfectly accurate.

"I never really though I'd end up living here," she said. But her husband was killed in World War II and she had an infant daughter to care for and England was rapidly becoming too depressing. In Washington, working for the war-time government, she met the man who was to be her second husband, Bob Treuhaft. She moved to Oakland, Calif., where she and her labor lawyer husband have lived ever since.

"I thought California would be glamorous," she said, but she has no sympathy with the hot tubs, gold bracelets and Valium life style of such places as Marin County. She pronounces sentence-"pixilated."

The Treuhafts set off in search of the communist party and were dedicated members for 15 years. She went to Mississippi in 1951 to plead for clemency for Willie McGee, a black man facing a death sentence for a rape conviction. She seems to have found pleasure in the most mundane of party tasks-from petitioning to picketing to distributing leaflets.

Her irrepressible humor occasionally got her called on the carpet by the more dour party members. At one point, she changed the name of some required reading from "Mastering Bolshevis,," to "Bolstering Menshevism," which, of course, is quite the opposite. The party did not take it kindly. 'Terrible Doldrums'

Several reviews of "A Fine Old Confliect" wondered how such a raucous spirit could submit herself for 15 years to the demands of party discipline. "It was quite simple, really. If there is an overriding principle or cause, you submerge."

The promise of the party had faded by 1958; she and her husband resigned. "I never dreamed my children would grow up under capitalism," she said, with echoes of the old amazement. But there was little time for disillusionment. "Things were stirring all over the place, the students were already getting active, there was so much to do."

Fifteen years as a card-carrying communist did not look wonderful on a job resume in the late 50s when Decca Mtiford was casting about for something to do. So she turned to an idea she had had since coming to this country.

"I thought it would be such fun to work for a newspaper," she said. "Every day one would be in touch with some sort of interesting reality."

As it happened, her husband was involved with a funeral collective in the Bay area. Mitford began to investigate the undertakers and wrote the book that polevaulted her to fame. The book led to articles, such as the one on the Famous Writer's School, and to books on the Spock trial and the prison system, the latter entitled "Kind and Usual Punishment."

But the frontier on which her battle with the marvelous idiocy of institutions was joined is harder to find now. The issues are fragmented, the super-structure of ideology and anger has crumbled.

"I feel like everyone that it's a time of terrible doldrums," she said, and she paraphrases a former activist who came to speak at a class she was teaching at San Jose College.

"In the 60s, there was a purity of motive, a belief in the goodness of people. Students were incredibly angry when they felt they had been lied to. Now, after Watergate, they expect to be lied to. Unless some organization emerges with some backbone, some viewpoint, everyone will continue to be groping. Perhaps, the reaction to the nuclear thing will provide an opening wedge.

"But then I am by nature, an optimist."

The leaders, present and possible, are dismissed scornfully. President Carter is "atrocious." The only good idea California Gov. Jerry Brown has had was to appoint her husband to the state board governing the funeral business. CAPTION: Picture 1, Jessica Mitford, By Harry Naltchayan-The Washington Post; Picture 2, Jessica Mitford, by Harry Naltchayan-The Washington Post