Mexicans have tended to be more faithful in this regard than others, but Mexicans are not by any means all of the Hispanics (20 million Mexicans vs. 15 million other Hispanics). The rates in Dominican and Puerto Rican neighborhoods are truly appalling. The Vatican needs to send over a new Mother Cabrini. A typical parish in a Puerto Rican neighborhood in Philadelphia has the following statistics. St. Hugh of Cluny - 15,200 inhabitants of whom 80% are Puerto Rican, 4,120 Catholics, of whom 90% are Puerto Rican. So just 3,860 of 12,010 Puerto Ricans participate in the life of the local parish - that's 32%, meaning 68% of the Hispanics are NOT Catholic. Of course, a simple drive through the neighborhood could tell you that, with the proliferation of Pentecostal and Evangelical churches.
However, even for the Mexicans, simple exercises can show the problem. The Catholic population of the Diocese of San Antonio is lower than the population of the overall mostly Mexican Hispanic community in the same geography. Considering that there are certainly some Irish, German, Italian, etc. Catholics in San Antonio, clearly, the number of Mexicans who are part of the Church there is no more than 50% or so.
The numbers put out by some Hispanic organizations, such as here are innacurrate, since they overstate the US Catholic population by about 15 million - that's 15 million people who are not registered with the Church and definitely not coming to Mass on Sunday. Most of the overstatement is among Hispanics.
You are forgetting about Jehovah's Witnesses. I don't know about your area of the country but in South Florida they are HUGE among Hispanics and Haitians. I figure at least 40% of Haitians here are JW. Interestingly there are FEW white JWs down here.