It's obvious from the title that hair – and a certain haircut – is of great import in this text. It sounds silly, but hair (both Bernice's and Marjorie's) has great symbolic weight here. There's a reason that everyone is so shocked and titillated by the idea of Bernice bobbing her beautiful hair; up until the 1920s, long, glorious, pampered hair was a key component of traditional feminine beauty. The idea of bobbed hair, which came into style in the Roaring Twenties, was considered scandalous and, as Bernice herself jokingly comments, even "unmoral" (72). At the end, one of the reasons Mrs. and Mr. Harvey are so upset by Bernice's bob is that one of their friends, Mrs. Deyo, is a strong opponent of bobbed hair.
The fact that a simple hair cut could so upset an entire town may seem ludicrous to us now, but if we consider it in the context of the changing social period Fitzgerald lived in, it makes more sense. Long hair represented both a woman's beauty and her virtue – and bobbing one's hair simply wasn't seen as something a respectable, well-bred girl would do. When Bernice bobs her hair, she is horrified by the dramatic way in which it changes her whole appearance; it robs her of her charm and renders her totally altered. This horror is what makes her ultimate revenge on Marjorie – the removal of her cousin's beautiful hair – even more fitting and gleefully, sinfully, delicious. Marjorie, for all her talk of being a modern girl, prides herself on her traditional feminine beauty, something that Bernice violently takes away from her at the end of the story.