In a way it does take away from the effectivness of the joke. On the one hand, it's always enjoyable to make fun of other's flaws. This has been a cornerstone of jokes since the begging of man.
On the other hand, the joke is about a man we rely on to lead us. And it's not making fun of a flaw like he's a klutz. ala Gerald Ford, but we're making fun of someone's mental capacity, who happens to be the President.
This is really depressing, that we think about the leader of the free world this way! Not that he should be sacred and beyond reproach, but if we don't think highly of our leader, how is it that this guy is still in office?
At this point the joke in on us, and there's nothing funny about it! :nonono:
