Is my brother calling Tapu Koko (the pokemon) mexican, racist?

why would it be, Ludicolo would be a better match,
im mexican and ludicolo is the most stereotypical mexican pokemon, next one is hawlucha.
tapu koko is a totem with hawaii inspirated colors so. no tapu koko cannot be considered a mexican pokemon

I don't know your brother, so I don't know if he's calling Tapu Koko racist or not.

You may want to rephrase your question.