I've had a Mexican roommate who's pretty sure he's 100% Castilian, and another Mexican roommate who had a striking resemblence to those Olmec statue heads (although he only spoke Spanish and English, and had no national affiliation other than Mexican).
There are some maps like this that can give you a general idea of what the ancestry makeup is.
https://upload.wikimedia.org/wikipedia/commons/thumb/9/90/Distribution_of_Indigenous_Peoples_in_the_Americas.svg/1600px-Distribution_of_Indigenous_Peoples_in_the_Americas.svg.png
I've had a Mexican roommate who's pretty sure he's 100% Castilian, and another Mexican roommate who had a striking resemblence to those Olmec statue heads (although he only spoke Spanish and English, and had no national affiliation other than Mexican).