Why does she have her mother's maiden name?

Probably because her Chinese-American father jumped ship and she & Meg were raised by their mother (along with a stepfather, who entered the picture at some point). They chose Tilly over Chan (their father's surname).

Who gives a shit? That's a personal family matter. Just look at & listen to her, and enjoy the pure sexiness. She's amazing.

MY LAST NAME IS MY MOTHER'S...EVEN THOUGH MY FATHER HAS BEEN MY FATHER SINCE THE BEGINNING...MY DAUGHTER HAS MY FATHER'S NAME THOUGH...SHIT HAPPENS.