2) He Uses His Look as a Crutch.
Appearing again and again as the same roly-poly, bushy-faced little man-troll has established Galifianakis as a brand, but being fat and furry is not the same thing as being funny. He keeps layering on the silly costumes — retro t-shirts, the famous man-purse in the first Hangover, the odd safari suit of Hangover II, and the sleeveless bush jacket he wears this time around — because his comedic talents are so feeble.
He’s essentially a prop comic, except he wears his props, and tired prop comedy, like the third Hangover film, is something that happens in Vegas and ought to stay in Vegas. Someone needs to tell Galifianakis that comedy movie stars have to give the audience a little bit more than clown wear.