It's everything it should be, and less: tasteless, tactless, graceless and gross. "ABC's Silver Anniversary Celebration" proves true at all that's wrong with, and irresistible about, that junk food for the eyes and ears that is American television. The show couldn't be any better if it were good.

ABC celebrates its 25 years of existence as the tawdriest and quickest-to-grovel of the three television networks Sunday night - at 7 on Channel 7 - with a four-hour marathon of film clips, self-congratulation, grotesquely slick production numbers, stars in gaudy abundance, and applause, applause, applause. If you took out the applause, it would be a three-hour show.

In 1953, the American Broadcasting Co. merged with United Paramount Theaters, Inc., the TV network thus born consisted then of only 14 affiliated stations. In the ensuing 25 years, most of them spent in raggedy third place, ABC did more than either CBS or NBC to move television away from New York, with its theater and radio traditions, and out to Hollywood, with its trivializing and homogenizing traditions.

Twenty-three years of attempting to find something beneath the lowest common denominator finally paid off and for the past two years ABC has been No. 1 in the ratings and the profits. In the process, it must be admitted, the network has also engendered some of the finer hours in commercial TV history, foremose among them the most-watched of all TV shows, "Roots."

A 10-minute recap of "Roots" near the end of the anniversary special is amazingly moving; every excerpted scene is charged with emotional energy. "Roots" is spine-tingling even in glimpses.

Only a few minutes before, the same video space is given over to the gunfire of "S.W.A.T.," the flutter of "The Flying Nun," and a scene in which faithful Indian companion Tonto hands the Lone Ranger his hat; "Me wash in stream . . . to make whiter." The guantum leaps from ridiculous to sublime are spectacular and stupefying; that it all levels out into something called television remains fascinating.

Executive producer, ironically or appropriately or both, was Dick Clark, who, like ABC master programmer Fred Silverman, recently defected from ABC to NBC. That Clark chose to open with Barry Manilow and close with Toni Tenille says quite enough about the Las Vegas mentality at work here. It's ideal; medium and message one and inseparable.

Genuine highlights include Alan King's funny and irreverent history of ABC programming, John Wayne's tribute to TV Westerns, and a medley of past musical moments that starts with Crosby and ends with Sinatra. People grow up, old, fat and gray before our eyes; there is a magic to television after all.

For the finale, members of the all-star audience come forward to join Tenille for round after round of the "Rocky" theme with new lyrics glorifying the net work. Silverman, interestingly enough, is never glimpsed during this hallelujah chorus, but ABC Chairman Leonard H. Goldenson and ABC President Elton H. Rule can be clearly seen.

The really, truly, stop-you're-killing-me exquisite moment of them all comes just before, when David Hartman raises a toast not to ABC but to the great god television - the mythic apparatus that had made stars or millionaires out of many in the room where the party was taped. Just as Hartman is pledging continued devotion in televisionland to "the benefit of mankind," there is a shot of Charo lifting her glass of champagne.

No one who cares enough about television to love and deplore it should even think of missing a minute of "ABC's Silver Anniversary Celebration."