Depends, but in general it's not legal. Mexico is wary of seeds entering the country, and extremely wary of any type of agricultural product sent to the US. It is possible to send commercial things (branded products, with manufacture details etc.) but artisanally prepared products can be difficult.
Also, there are very few seed houses in Mexico, and they mostly import their seeds from the US, China and Europe. Rancho Los Molinos is one of the largest companies (they wouldn't send to the US when I tried).
Many afficionados (like me) source their seeds internationally. The offer of national vendors is often limited and not always reliable. There are a few groups on FB though.
If you look for "genuine Mexican pepper races", then the situation is very deplorable. Every region has its own varieties that are almost impossible to get unless you visit the region yourself - even for Mexicans - or know some one from the region. Academic reports on the subject are poor and often only available in Spanish. CICY (Yucatán) is the only exception.