There’s actually many varieties of garlic grown in Mexico (e.g. Chileno garlic), and “Mexican Garlic” isn’t even the name of a specific, agreed-upon cultivar of garlic.
Some people equate it with Spanish Roja Garlic. Others equate it with, and sell it as, a garlic that has fat cloves with pinkish-purple skins, and a mild, somewhat sweet flavour.
This last garlic is used in Tex-Mex and Southwestern cooking in America.