It was clear Bush was dim before he became president:
Jacob Weisberg presents his top 25 Bushisms of all time
. Of interest is that 10 of the 25 were from the year 2000 (before the election).
I've heard it said that English is practically a second language to GWB, his native tongue being Texan.
They were funny in 2000, maybe early 2001. But after Bush has murdered tens of thousands of people and ruined (for generations) the lives of millions more, I can't really bring myself to think of him as a punch line anymore.