Maggie's smile represents a victory.  Throughout the story, Maggie never seemed to be happy and was always hiding in the shadows of her sister.  When her mother told Dee that the quilts were for Maggie, she was surprised and smiled because, for once, she was able to receive something instead of her sister.  Her sister turned the quilts down when she left for college because she felt they were old-fashioned. Now that she is all into showing off her heritage, she wants them to display. She always felt like Dee got everything and she even said, "She can have them Mama."  She didn't need the quilts to have the memories.  Dee says, "Maggie's brain is like an elephant's."  This comparison indicates that elephants have good memories and Maggie would use those memories and put the quilts to "Everyday Use."   When they walk Dee to the car at the end of the story, Dee tells Maggie she should make something of her life and insinuates that as long as she and her mother stay in that house they will never improve their life.  Maggie smiles because she knows they have the best life of all.  They respect their culture and memories and know that memories are meant for "Everyday Use" so one can always remember the past.

Reference:  The Language and Literature Book by McDougal Littell 

It means that even though she is disabled mentally, she is aware of the situation enough to know that she finally got something that Dee/Wangero wanted. She has been ignored and invisible her whole life. But here in this showdown of wills and ways, she was awarded a prize over her sister. A rare victory.