According to the evidence in the story, Mama is correct.
Throughout the story, Dee had been offered the quilts before and rejected them as old-fashioned. If this is about being right about the quilts, she's changed before and will change again (the story suggests).
Mama knows the history of the quilts; Dee has worked to forget that history, even down to changing her name.
When they are arguing, Dee looks at Mama "with hatred." That's not a character marker for doing the right thing.
Most importantly, though, Mama has a realization like a touch of the divine that what she's doing is right.