"Mommie Dearest," the film version of Christina Crawford's poison-pen memoir of her adoptive mother, Joan Crawford, looms as wretched excess. Considering the source, however, this ill-advised and disreputable movie could have been worse.

It's apparent that the movie wasn't conceived as an unmitigated hatchet job. The crisp, deftly suggestive opening sequence depicts the actress arising at 4 a.m., going through a Spartan beauty regimen (for example, after splashing her face with scalding water she buries it in handfuls of ice cubes), riding to the studio and being made up for the first shot of the day. Throughout the sequence one sees only fragmentary glimpses of the actress' face. She is revealed in full face and makeup in the concluding image: There's Faye Dunaway in her Joan Crawford disguise, and the resemblance is fairly amazing.

Unfortunately, the impersonation proves a triumph of droll costuming, makeup and hair-styling rather than inspired, sympathetic acting. Even the superficial disguise Goes Too Far, degenerating into a running gag sustained right through the shot of Crawford in her casket: Dunaway seems to be sporting a new gown and extravagant coiffure at every entrance.

Already tarnished by Christina's book, Joan's memory may be condemned to terminal ridicule by Dunaway's grotesque performance -- a terrible performance for the ages.

Although the book is coldblooded and spiteful, it's sadly conceivable that Joan asked for a good deal of the posthumous abuse dished out by Christina, the eldest of her four adopted children. You don't need to rely exclusively on Christina's embittered and often ludicrously written accounts of her wrathful "mommie dearest" terrifying the kiddies with drunken rampages and severe punishments for trivial infractions of fanatic rules of neatness and decorum. A sympathetic biographer, Bob Thomas, reached a harsh verdict on Crawford's maternal performance, closing the subject with a quote from one of her best friends, Helen Hayes: "Joan tried to be all things to all people. I just wish she hadn't tried to be a mother."

Some corroboration was also supplied indirectly by Joan herself in a recent collection of engaging, candid and revealing interview material called "Conversations With Joan Crawford." For example, the following remarks certainly suggest an awareness of failure:

"I really don't think the stars of my time should have had children, whether we bore them or adopted them . . . If you were working -- and I worked almost constantly while the children were young -- you got up at the crack of dawn five or six days a week and came home at dusk, if you were lucky. You didn't see your kids in the morning and at night you were so goddam tired it was all you could do to smile and kiss them good night. On the weekends you were tired, exhausted, absolutely shot, and you'd have welcomed some quiet hours with the kids, but usually there were the social things you had to do for the studio, for your career, and sometimes, but so rarely, just for and with friends . . .

"I think what it boils down to is the fact that a part of us wanted a real, personal, private life -- husband, kiddies, fireplace, the works -- but the biggest part of us wanted the career, and that biggest part had to live up to the demands of that career."

If it exposes anything, "Mommie Dearest" exposes Dunaway as a preposterously limited acting instrument. It's amusing to watch her in several scenes with Mara Hobel, who plays Christina as a little girl. Hobel has an economy of expression and a certain stillness that Dunaway conspicuously lacks. Sucking in her hollow cheeks, flaring her eyes and twitching her facial muscles this way and that, Dunaway makes you intolerably conscious of what a diffuse, jittery bundle of nerves she is. When she bellows at the kids during her mad scenes -- in the maddest of all she's smeared with cold cream, which puts you in mind of a crazed Kabuki -- you detect something else disillusioning: That voice isn't equipped to bellow. It thins into a fragile croak just when it ought to be scaring the daylights out of you.

Maybe that's the key to her limitations. Dunaway has a certain high-strung, neurotic intensity, but when she's encouraged to go for big histrionic effects and pull out all the stops, you discover that she can't help but play thin. She's not a power hitter. Even certain things about the disguise begin to nag at you. For example, those flaring eyes are wrong somehow: They don't open to the camera with the luminosity Crawford's eyes projected. And is that really Dunaway impersonating Crawford? From certain angles Dunaway looks more like Lily Tomlin doing a Crawford impersonation.

The would-be sensations of the show, the big domestic horror scenes, sustain no sense of horror whatever. The audience perceives them as outrageous domestic slapstick, a perfectly justifiable misconception and the decisive indication that director Frank Perry has lost control of the show after that surprisingly astute beginning.

In one donnybrook Dunaway brutalizes Hobel when she discovers a wire hanger -- strictly forbidden by fastidious mommie -- in the little girl's closet and pounds the kid with the vile hanger (the blows suggest a heavier weapon, maybe a blackjack or telephone pole) and tries to roar. In the second she tries to strangle Diana Scarwid, who has entered to play the adolescent Christina. It was impossible to judge which episode provoked more uproarious hilarity.

Of course, it would have required prodigious sensitivity and discretion to depict the Crawford menage as described by Christina without wallowing in farce. Moreover, one doesn't envy screen writers obliged to hack a playable, coherent continuity out of the complicated chronology and simple-minded psychoanalysis that clogs the book. It's a booby-trapped source, and there are intermittent signs of both skill and wariness in the filmmakers. The astuteness shown in the opening sequence lingers for a while. Joan's professional situation gets a little more attention, and the writers try to account for the longings that lead her to adopt Christina.

But once the ugly stuff begins, all that methodical preparation and desire to be fair becomes meaningless. The movie is committed to a prolonged, exhibitionistic wallow and can't escape the trashy consequences.