There's a difference between using an inappropriate word but describing the same phenomenon, and treating an ethnicity as synonymous with "poor".
It's both, but as an ethnicity it's subdivided in several cases based on their origin (e.g. "white mexican").
But can also be seen as the future/aspiring all-encompassing Mexican ethnicity through the inter-mixing (cultural and genetical) going on for centuries...
(It's not like "Spanish" in Spain are some pure ethnicity that always existed either, it's one that emerged by combination of several ethnicities).