How bad is it? It's not the worst Wayans brothers film, but that's faint praise indeed.
Should you see it? I can't see why you would.
Laurel and Hardy did the adult-as-baby thing in "Brats," so updating that could've been funny. Even Bugs Bunny had "Babyface Finster," a similar idea, and that worked. Here, an adult's face is placed by CGI onto a child's body and there's some funny bits (but not enough for a feature film). The film reportedly cost $60,000,000 to make, the preposterousness of which is exceeded only by the fact that the movie made money ($100,000,000 gross). The plot: a diminutive criminal tries to recover a stolen diamond stashed in the house of a couple that want a child, so he pretends to be one. Babies are inherently gross, so breastfeeding jokes and diaper jokes are expected, but guys get hit in the crotch at least a dozen times. The film derails when it becomes a "Home Alone" clone with toys as makeshift weapons. Besides the Wayans family and their usual cast (David Alan Grier, Kerry Washington) one finds Chazz Palminteri, Tracy Morgan, Molly Shannon, Fred Stoller, Alex Borstein and Rob Schneider (uncredited as Dinosaur Rex).