Looking strictly at "nationality" would be problematic anyway. There are plenty of largely Euro-descended Mexicans, educated, skilled, civilized, who would likely assimilate into American society in a heartbeat. Then there are the legions of Mestizo and Central-American stock from Mexico(or Guatamala, Honduras, Peru, etc.), who are of entirely different ethnic extractions, and inclined toward a far more primitive existence in their home territories. Thus, a "Mexican" can mean many things and any hopes for easy or even feasible assimilation are effected greatly by this diversity.
This is also why many "immigrants" are broadly and misleadingly categorized as "Hispanic" which is an essentially meaningless designation today. A "Hispanic" may theoretically refer to a well-to-do, green-eyed, blong-haired Argentinian, or a dark-skinned, diminutive Mestizo Amerind, who is more Asiatic than anything else...so long as they both speak some derivative of the Spanish language.
"Hispanics" now make up the largest minority group in the US(some 15%), and the bulk of them are recent arrivals, here illegally(or at least enteretd the country illegally), poor, illiterate, etc. That is a massive demographic shift that has expanded the nations underclasses exponentially in just a couple decades. These "immigrants" are often exploited, the American people are exploited...and Wall Street, The Media and the Federal Government look askance, even as they praise this great and dangerous change in the historic makeup of the country, while they increase their collective power and bottom lines.
Look to Southern California to see your future even now America.
Huge disparities in wealth, third-world crime rates, unrest, inter-racial violence(black vs. hispanic), unemployment, failing schoolsystems/record dropout rates, general poverty, and an insolvent state government.
Does that sound like sound immigration policy to anyone?