in everyday use ( why does Dee think mama and Maggie don't understand their heritage ?

It would seem, on the surface anyway, that Dee sees her culture from a sanitized superficial way.  Dee's camera shots are as much a reflection of Dee’s rebuke of her culture as they are of chronicling it. She thinks Mama and Maggie consciously refuse to culturally evolve. Dee does not understand that Maggie and Mama's life are her culture.