To inform the reader of Mrs. Flowers's social position.
By pointing out that Mrs. Flowers was "the aristocrat of Black Stamps", Angelou wants to draw a parallel between the uneducated and underprivileged society of black people and the "normal", white world outside. Black people can also have aristocratic manners - if and when they are provided with the means to educate themselves. Aristocracy is here a metaphor for all the privileges that black people were deprived of. A little girl cannot think in concepts, cannot identify the roots of her obsession with her role model, but simplifies it and interprets it as gentility. Mrs. Flowers is everything that she wants to become when she grows up.
