I really enjoyed reading your story about your grandmother. I could relate to your story on personal level because my family also moved to America when I was younger. We faced many kinds of treatment because of how we looked (Asian) and how we talked.
I agree that Tomas trying to look like Mexican is because he wants to look rough but I do not think that means he is ashamed of being half American half Filipino. Being the man of the house after his father left and doing the job he does, he does not want people to think he was weak. His transformation was a choice he made to survive in society. People usually have these level of how they view Asian people, Hispanic people, and Black people. Usually the Asian people are viewed as not a threat or friendly/weak. I think the narrator presented the same idea when he was introducing his brother in the beginning of the book. The buyers which mostly consisted of rich celebrities like to buy these guard dog from a person which they think are though. So I think Tomas chose to change his physical appearance to survive in society.