I think there's a fine line between punching bag and comic relief, and that line was broken long, long ago. Some punching bag characters that come to mind are that Redhead from Bill and Mandy, Sokka from Avatar (40 percent of the time), Candace from Phineas and Ferb, as well as the professor, who's name I can't spell (Candace's Mom thinks Candace is insane, while the professor's flashbacks are good enough proof), and yeah that's about it.
That Redhead, because he usually gets victimized out of nowhere.
Sokka, because he can't bend anything, as well as lack of respect in earlier episodes.
But yeah, it's seems to be funny, but I can't help but pity these punching bags.