While we can be confident that Fox wants to put the best spin he can on Mexican contributions to the country, we can assume that he knows what he is talking about on this subject.
"Oh, starting with Mexican food! The jalapeños and the tacos and the rest. I think they have contributed family values. And then we have our culture. When you were killing Indian Apaches there, we had built Mayan cities, the pyramids, Mexico City."
A well-informed Mexican lets us know that Mexican Americans have given us good food and family values. Perhaps Mexico has benefited from its Aztec past, but I don't see how that is connected to improving America. And the "family values" cliche is just a neocon talking point. American families are not not stronger because of some Mexican influence.
So, based on Fox himself, Mexicans, compared to other American groups, have not contributed much to the country. Don't get me wrong--every American who lives a decent life is contributing to the well-being of the country, but "cultural contribution" is a different question.
General Social Survey respondents were asked, "Since the beginning of our country, people of many different races, nationalities, and religions have come here and settled. As I name some of these groups please tell me if the group has made: 1) one of the most important positive contributions to this country, 2) an important contribution, 3) some contribution, or 4) little positive contribution to this community."
Respondents were asked about many groups, and Mexican Americans were on the list. I identified factors predicting that a person thinks Mexicans have done a lot by estimating a model with ordinary least squares regression. If a factor, for example sex, turned out to be unrelated to one's attitude, I removed it from the model. Here's the final list:
Thinking Mexicans have contributed a lot (OLS regression coefficients--standardized, N = 1,033)
Job prestige .10
Church attendance -.05
Political conservatism -.08
The profile of someone who gives Mexicans a lot of credit looks like this: young, educated, prestigious job, not religious, and liberal. Objectively, this immigrant group has not contributed a lot compared to others--even Fox doesn't make a contrary case--but SWPLs, supposedly our best and brightest, are most likely to view Mexicans as playing a key role. If a SWPL feels strongly about something, the smart move is to bet the other way.