There are no doubt hundreds of frustrated chorines who could sing the three Sondheim originals "better" than the most famous person in the world. But with its pedigree of wit and musicality, show-tune pop-schlock sure beats the direct-to-Vegas power ballads with which she's heretofore betrayed her dance-rock roots. Especially when she writes it herself--except for the "Material Girl"-inspired "More," the Sondheim tunes are fussy and genteel (with Mandy Patinkin's "well-sung" cameo the nadir), but such fake period pieces as "Cry Baby," "He's a Man," and the risque s&m-lite "Hanky Panky" are all her. This is a woman whose great gift is for the mask. Camp isn't everything she can do, but she sure knows how to do it right. A