In "Everyday Use," is Dee wholly unsympathetic? Is the mother's victory over her altogether positive?

What an interesting consideration! 

As to Dee:  first, I think it is important to note the date of publication of Walker's story, 1973.  This was a time when African-Americans were truly starting to discover their African heritage.  Magazines, novels, newspaper articles all celebrated the uniqueness of African life and urged readers to embrace their authentic selves.  As a young woman trying to find her place in the world, such rhetoric had vast appeal.  We want to feel we are special.  Furthermore, it is not unique to shove away the familiarity of our parents in favor of something "new."  No doubt, were we able to "speak" to Dee in the future, she will have mellowed and found the value both in her own life and in her heritage.

As for her mother, she does miss out on some things:  knowledge of her deeper roots and an education.  These impediments create a rift between Dee and herself that may never be fully overcome.  When a child and mother are so at odds, a victory can never be seen as complete. 

Here is Dee's line at the end of the story:  "Your heritage," she said, and then she turned to Maggie, kissed her, and said, "You ought to try to make something of yourself, too, Maggie. It's really a new day for us. But from the way you and Mama still live you'd never know it."

It truly was a new day for appreciation of roots.  The problem is in going too heavily in one direction or another.