ACE VENTURA: PET DETECTIVE (PG-13) If you thought movie comedy had scraped bottom with the Ernest series, check out Jim Carrey (of TV's ''In Living Color'') / as Ace Ventura, pet detective and postmodern doofus. His ''characterization'' consists of strutting around in a Hawaiian shirt and Li'l Abner pompadour, flashing his big, creepy-preppy grin like a junior Richard Nixon on Jolt cola, and punching home every last one of his so-stupid-they-really-are-stupid lines. Carrey suggests an escaped mental patient impersonating a game-show host and, what's worse, his hyperbolically obnoxious shtick is the whole damned show. F

