I don't recall anyone saying that "all Mexicans wear those hats". But to see this image of a man with Iberian features and a hat with an enormously wide brim, it's not unnatural to make the connection that it could be a sombrero, and therefore the man could be Mexican. Personally, I don't think it does look like a sombrero, but then the first time I ever saw that picture was in the context of the preview I linked to. If I had been shown that picture without knowing who all the new civs were, and was told, "This is President Juarez of Mexico," I would probably have responded with, "Is it really? I don't remember him having a mustache."