How much does Mama's perspective perhaps color (or not) the reader's impression of her daughters in general and on you, as a reader?
Mama's perspective certainly influences the reader, both in general and in particular, to view her daughters in a particular way. Maggie is the victim, the shy one; she "will be nervous until after her sister goes: she will stand hopelessly in corners, homely and ashamed of the burn scars down her arms and legs, eying her sister with a mixture of envy and awe." Mama describes Maggie as slow and sort of pitiful, like a lame dog that hopes for kindness from anyone. We are thus prepared for Maggie to be weak, without really having a sense of her goodness.
On the other hand, Dee, Mama says, "used to read to us 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 necessarily need to know." It seems that Dee has attempted to assert her intellectual superiority of over her family for a long time, and given Mama's dream of a television reunion, it sounds like Dee has kept away for quite a while, likely out of a sense of embarrassment. Therefore, we are prepared for Dee to be a little standoffish and even snobby when she arrives. It seems as though Mama, then, paints a more accurate picture of Dee because she doesn't really seem to appreciate Maggie's merits until the story's end.