That is even less likely than Germany winning in Western Europe. Even with a German victory in Europe, the US military would be able to crush the Mexicans (the German navy would not be able to project power across the Atlantic with sufficient strength to be able t provide much support to the Mexicans, plus they would be busy occupying their conquered territories).
If by some chance Mexico could defeat the US, they still wouldn't be able to rule control the large white, English speaking population. Maybe martial law would do the job for a while, but that isn't a long term option.