Is mia tsu?
it means something like "chic/hip/fashionista", i'd say "no" is the answer, but based on context, im not sure he is using the term correctly, i dont think it would be possible to interpret Mia as tsu, other girls maybe, but not mia.
maybe he's trying to abbreviate tsundere (temperamental and snobbish)? again, i'd say she is not.