Would they have to? The USA has a GDP (PPP) per capita around 3x that of Mexico. The sheer magnitude of the difference in purchasing power would make patio11's claim true, even if they were hypothetically, in our president's words, sending us drug dealers, criminals, and rapists.
I'm not sure exactly what you are arguing for here, so please correct me if I'm wrong. You seem to to saying that the middle class in Mexico are not the same as middle class in the US because their wealth + income would not be regarded as middle class by US standards.
I don't think that is a good argument, since once they move to the US they would be making middle class wages in the US, since they have the relevant skills to be employed in middle class jobs.
