"I find it funny that the same people that claim Bush is an incompetent idiot also seem to claim that he can destroy the economy not only of the US but of the entire world. There seems to be a conflict between the two concepts."

Can't see the conflict, Mario. You don't have to be smart to set fire to a neighbourhood while you're trying to cook your dinner.

Now if the suggestion was that he was intentionally setting out to wreck things, there might be a conflict, since achieving what you are trying to do is the essential measure of competence. But accidentally screwing things up and having them blow up on you, that doesn't require competence.