Can a hippopotamus be a fireman? No. . . . A piano player? No. A cowboy? No."" And so Mike Thaler ticks off a total of thirteen occupations, one for each double page, as Robert Grossman's bold, super-realistic dream figures make clear why each is closed to hippopotami. Whether the pictured hippo breaks the ladder as a fireman, crashes through the floor as a ballet dancer, or gets an arm stuck in a tall hat as a magician, his bulk is invariably the disabling handicap, and by the time he finally gives up (""Until I think of something, I'm just going to lie down in this hammock and take it easy""), it's clear that he hasn't much versatility as a comedian either. And if the title question is never answered, neither is there a clue as to how, in these days of retrenchment, such a klutz rates the zappy color treatment.