It is quite possible that George W Bush suffers from mild aphasia, which causes some of his words to come out awry.
But in a number of the quotations classed as 'Bushisms', he is clearly poking fun at his hearers. The fact that they don't get it adds, I think, to the amusement.
He is of course far better educated than the majority of his critics, with a bachelor's degree from Yale and an MBA from Harvard; despite what anyone may claim, those institutions don't accept students who don't qualify to study there, and don't hand out degrees to people who don't deserve them. If they did, their own reputations would be worthless.
Bush will acquire a much better reputation when his presidency is put into historical perspective. Perhaps one day too the BBC, the Independent, et al will get tired of sneering at him.