It's hard to go into the failures of Mexican athletics without treading into certain unsavory racial territory but they are the most obese countries in the world (75 percent of the population is overweight or obese), have acute struggles with diabetes etc. There are not a very tall country either as the average Mexican man is about 5'7.
Having traveled throughout Latin America fairly extensively I can say Mexico is way less superficial than a lot of South American countries but that also has the downside of working out not being taken very seriously in the country. Working out has become really popular in a lot of South America over the past few decades but still isn't as much as a thing in Mexico.
I'm sure with a better foundational setup Mexican futbol would be in a better place, but I think there is probably a ceiling to how good they'll ever be due to the physical limitations of the bulk of the population.