It could give popcorn a bad name-again. Starting this month, 1 million popcorn bags at Cineplex Odeon theaters nationwide will promote New Line Cinema's Dumb and Dumber, starring Jim Carrey, and Don Juan DeMarco, with Marlon Brando, Faye Dunaway, and Johnny Depp. New Line spent about $60,000 on the gimmick, which marketing president Chris Pula says ''is a lot of bang for the buck.'' Sharing space with a Carrey film will give Don Juan a boost, but what could Brando's mug do for snack sales? ''Yes, he has a weight problem,'' says Pula, ''(but) we darkened the side of his face and toned his weight down.'' Butter on that?