Does Mrs. Johnson criticize Dee? Maggie? Why? Does she fairly and accurately describe both daughters? How does this help to establish the attitude toward heritage?

Mrs. Johnson does not explicitly criticize her daughters, but I think it is obvious that she is aware of their flaws. For example, she never calls Dee selfish or cruel, but she says that Dee

used to read to [them] without pity; forcing words, lies, other folks' habits, whole lives upon us two, sitting trapped and ignorant underneath her voice. She washed us in a river of make believe, burned us with a lot of knowledge we didn't...need to know.

Her criticism of Dee seems implicit in passages like these. She says that Dee has a "scalding humor" that seems to burn others, though she never actually calls Dee malicious or mean. Likewise, Mrs. Johnson does not explicitly criticize her other daughter, Maggie, either. However, again, she seems to accurately describe Maggie in order that we may draw our own conclusions about her. Mrs. Johnson says,

Have you ever seen a lame animal, perhaps a dog run over by some careless person rich enough to own a car, sidle up to someone who is ignorant enough to be kind to him? That is the way my Maggie walks.

She later describes Maggie as having a kind of "hangdog" look about her. She does not directly criticize Maggie but, rather, simply describes her to us.
Because of Mrs. Johnson's apparently accurate descriptions of both daughters—their accuracy seems confirmed by the women's behavior in the story itself —readers can see that we are not supposed to agree with Dee. Even with her mother's relatively impartial descriptions, it is clear that Dee is an unsympathetic character, while Maggie, on the other hand, is incredibly sympathetic. Thus, when faced with their differing views on heritage, it seems pretty clear that we are supposed to agree with Maggie's (whose view, I would add, is similar to her mother's, a woman who garners our respect throughout the text).