Its actually not just cartoons. It seems really that a LOT of actors (possibly especially comedians) are from Wisconsin; this may hold true as well for writers, so that's why they put that in a lot. No kidding - I had a roommate from Wisconsin once and he hyped the place so much all the time, making fun of California - it really burned me...but after that I always notice when Wisconsin is mentioned on television, and it is a LOT!! It's the darnedest thing.
Perhaps everyone from Wisconsin has a tendancy to make fun of other places and glorify Wisconsin whenever they're visiting/moving elsewhere, so much so that writers just can't HELP making fun of Wisconsin, just for revenge! That roommate of mine was pretty mean about California, and always going on about how great Wisconsin is. Maybe all Wisconsinites, or a lot of them anyways, are like that, and it just annoys everyone.
It certainly annoyed ME.