Matthew Yglesias points out a British political cartoon that's not funny. I mean, it's really not funny. See for yourself. This cartoon is a one-panel clinic in how to not be funny: make your joke so obscure that your point is undecipherable, engage in caricature for no particular reason, and invoke the dead, just for starters. Wow.
Matthew makes the point that a lot of times when people say, "I don't think this cartoon is funny" what they're really saying is, "I don't agree with the political viewpoint being expressed here", which is probably true in a vague and general sense. But there's a lot of political cartooning I don't find funny on the liberal side, and the only conservative political cartoon I personally know of -- that Ramirez guy, who I believe cartoons for the LA Times -- I actually do happen to find funny on occasion. Those are probably exceptions that prove the rule, though. I'm not sure if I have a point, so I'll stop now.