The body-snatchers are back, and this time they want our brains, too. Looker, written and directed by Michael Crichton, is about a sinister corporation that believes in the perfectability of commercials. Equipped with computers and other sinister devices, they transform a bevy of beautiful women into life-size mannequins, and use them in a dastardly ad campaign that can brainwash TV-viewers into doing whatever they're told. Fortunately for all of us, this scheme is squelched by our hero, a plastic surgeon played by Albert Finney. But before he saves the day, we are subjected to 90 minutes of the most foolish entertainment ever perpetrated by a recognized moviemaker. This is too bad, since the film's message - that the power of TV may easily be used for ill purposes - is critically important. It deserves a far more dignified forum than this messy movie, with its careless pacing, gigantic plot-holes, and exploitative use of actresses, set to a musical score that lurches from Vivaldi (yay!) to disco (boo!) and back again. Crichton has toppled a long way since the intelligent and progressive ''Coma.'' Here's hoping he finds the right track again.