Maggie is intended to be a sympathetic character, and so is presented in a much better light than Dee. Maggie is described as not very smart, but a hard worker. She is shy because of her burn scars, and doesn't like confrontations. In fact, the only explicit quality that she shows that might be considered negative is envy:
...eying her sister with a mixture of envy and awe. She thinks her sister has held life always in the palm of one hand, that "no" is a word the world never learned to say to her.
(Walker, "Everyday Use," xroads.virginia.edu)
She is jealous of Dee because Dee was very outgoing, so she seemed to have an easier time in her life. In fact, Maggie was simply too shy to risk being humiliated in society, and so kept to herself; Dee's "success" of which Maggie is jealous is actually just Dee's movement from her old family life to a new life of her own making. If Maggie is jealous of Dee's success, it is because she doesn't fully understand how much Dee has rejected her own heritage.