Oooh, that cat shirt goes well with the rest of her design! Indigo seems to work quite well with her mostly pink and yellow palette.
She also looks smug. Maybe it goes well with her personality? Maybe it's tied to one of her spirit animals? Maybe it's both.
Either way, SHE LOOKS AWESOME! And the extra bit of background info you gave her is a nice touch. Adds some depth to an OC with an already neat design!