It is widely known that most, if not all, vegetables have intelligence that, compared to humans, is close to zero.
I believe the woman is intelligent enough to know that president George Bush has an intelligence above zero. Obviously she does not rate Bush nor his brains highly, so logically seen, my guess is that she used an exaggeration, an artificial comparison, with the intent to show that Bush is painfully stupid. Hence, compare Bush's mind with a tomato. In which the 'a' of tomato is to be pronounced as the 'a' like they use it in Britain. Stick your hand in your mouth and try to say the letter a, then you know.

[/sarcasm]