It's seldom that the New Yorker is in the general news, except as the signifier of extreme high-browedness. But today it is, because of the cover depicting the Obamas in the White House.
My first reaction to the cover was that it obviously tried to ridicule the views of the extreme wingnuts concerning Obamas (that he's a terrorist Muslim, that she hates America etc.), but that it failed to do so. The reason for that failure is the same which made me once scrap a long funny piece about the wingnuts giving advice to America's mothers: I couldn't make any of the advice more extreme than what the wingnuts actually say.
In short, using exaggerations as a way into sarcasm about the wingnuts is almost impossible, and that should tell you how extreme they are.
Another way of saying that is Atrios' comment:
The New Yorker cover could have worked if had made more clear who it was satirizing (Fox news, the Republican party, Rush Limbaugh, whatever), or by being clever enough to provide the actual funny. As it is it's just a reflection of the Right's view of Obama, but there's nothing clever or funny about it. The cartoon could run as is on the cover of the National Review, also meaning to be "funny" but with a different target.
And the result is that both McCain and Obama have expressed strong outrage about the cover. Note that this is not because the cover was ridiculing the wingnuts, nope.
In that sense the joke certainly misfired.