I think Miss Fancy is a way of foreshadowing bigger and better things coming to Whistle Stop. She represents how as the times are changing and years progress, Whistle Stop has become more up and coming. Having her come to their tiny little town foreshadows new people, new stores, and new ideas that will soon come to Whistle Stop.
Good point Nicole ... Miss Fancy may be the symbol but Idgie is the one who is responsible for bringing her to the town so I also see it as another indication of the "magic" of Idgie. She can make things happen, and she cares about everyone (even little Naughty Bird).