I think it has something to do with living in a third world country.
My friend says that when he agreed to build a glass factory down there, he told them he was going to need an office on the factory floor. So, since he was designing everything, he drew up plans. When the guy delivered the lumber, it looked too *big. and had these really keen edges. Took him 20 minutes to realize that he'd specified 2" x 4" timbers, and somebody had taken some logs and milled them to his precise specifications. Which was really weird, because in lumber, an inch isn't really an inch. But how were they supposed to know that.
It's not a matter of whether they're smart or not, they're just in an entirely different situation where they have some big problems to worry about, like whether they'll get enough to eat, and maybe they don't have the opportunity for education. I hear you can buy penicillin capsules in the pharmacies like they were throat lozenges or something - an attractive box in a display next to the cash register. Probably not a good idea from the perspective of the problems you can get into with resistant bacteria, but, well, things are different down there.
Miguel de Icaza is proof enough that mexicans can be plenty smart, with the right opportunities. I've met him, he's far more intelligent than i am.