I think anyone that cares already knows that.... how many times do I have to tell people that Taco Bell is *not* Mexican food?
The mistake may have an historical origin. Taco Bell came onto the scene where I lived when I was starting high school (ie. a long time ago). At the time there were precisely zero Mexican restaurants (or people) around and nothing else even close. People needed a simple way to describe the newcomer, and somehow "Mexican" was the word people used. I don't know if "Tex-Mex" was too nuanced or complicated, but nobody had ever heard that term at the time. We had Italian, German and French restaurants, and so Taco Bell was deemed "Mexican".
Another way of looking at it is the Venn diagram approach. Taco Bell food is none of the other nationalities named above but you could argue that the TexMex circle overlaps the circle called Mexican, and might even be inside of it from the perspective of a northern Yankee. Tacos and burritos didn't overlap any other circle we were familiar with.