Both dopey and insinuating, Beck's full-length debut Mellow Gold is total novelty pop, a cheeky mix of Ray Stevens, Falco, and the Beastie Boys. His hit single, ''Loser'' with its acoustic slide hook, loping hip-hop beat, and antifolkie aura might have been dreamed up by a cynical ex-hippie exec to play over the closing credits of Reality Bites. And ''Beercan,'' a white-boy rap with bong loads of studio noodling, proves the winning formula isn't just a fluke. B

