Yes, how come it's suddenly okay to make fun of "big, sweaty fat people"?
I think the whole issue of humour and what "makes" something funny is enormously fascinating, and just attended a really interesting session at a (medieval, but the topic transcends chronological periods, really) conference on this very subject. The usual argument is that there are several "types" of joke/humour:
~ the sort in which there's an assumption of superiority on the part of the hearer/teller; ie, we're laughing "at" someone or something
~ the sort in which the humour is based on incongruity: a pun that makes a word mean something other than what's usual, an unexpected occurrence, etc. This is a "victimless" humour, even a baby laughing at a game of peek-a-boo can fall into this category...Mummy's ther, now she's not there, now she's back...
~ and then there's just pure pleasure: the laughter that happens watching kittens play, or flying a kite, just having fun, etc.
In a way, I think it's just the (occasional, as I *do* revere language & its possibilities!) inadequacy of words that allows us to use the same term to describe these very diffrenet things...the fact that the end result of each is often laughter is in many ways the only thing they have in common.
But it can't be denied that a good laugh is one of the very best things about the human experience, although we may have a wide variety of ways of finding one.