I'd call "Comedy" a genre of its own, although it usually overlaps with one or more others, eg Paranoia is some kind of scifi/scifan, TOON! is Cartoons (another missing genre with other reps like Cartoon Action Hour), and Tales From the Floating Vagabond is as much of a genre-blender as Rifts is. It's a bit more than just a play style, given that whole games are designed around the comedy instead of just having supplemental stuff like Sentinel Comics' Guise Book.
Heck, Fiasco is usually a comedy game in practice, even if the humor tends to go very dark.