Some of my favorite funny ones:
Dancin' Fool, as mentioned above, the whole thing is over the top.
Catholic Girls from "with the tiny little mustache" to "How do they go? Aaaaall the waaaaaaaay" and every other lyrical gem, this song is one of my favorites.
Wowie Zowie is somewhat more obscure, but was one of my first favorite Zappa songs, maybe because of the "I don't even care if you shave your legs" line and the Frankie Valli falsetto at the end.
You Are What You Is, love the grammatical jokes and the whole "traded his daishiki for some Jordache jeans" stuff.
Cosmik Debris contains perhaps the funniest line: "Now is that a real poncho or is that a Sears poncho?", as well as the hilarious "The price of meat has just gone up, and your old lady has just gone down" right before it. Brilliance.