When Lenny premiered last fall, it was an oasis in the sitcom desert, a raucous, juicy show about a Boston working-class family headed up by a real-life Bostonian, Lenny Clarke. But poor ratings compelled CBS to yank Lenny from its schedule for retooling. The show recently returned, on a new night: Saturdays, at 8:30. Of all the stand-up comics who've made the transition to TV recently, Clarke is by far the best actor, much more natural and expressive than, say, Roseanne Barr or Jackie Mason. But in the new version of Lenny, Clarke's character has been softened; instead of being a well-meaning lout, more often than not he's just a sentimental dufus. Moreover, the scripts have lost their edge; a recent show about an interracial marriage was overwhelmed by its plea for tolerance. It's one thing to express a noble sentiment; it's another to do it at the expense of your characters. Half the cast suddenly turned into bigots who opposed the marriage of Lenny's sister to a black man; the other half became pious saints. Neither attitude suited a show that's always offered subtler shades of personality. Gee, CBS, if you're going to kill Lenny off, at least let the poor stiff die with dignity.