This is a blackly humorous tale that reflects some of the cultural mores and expectations of the day. The title is very clever, as it can be taken in two distinct ways that reflects the two incidents of hair "bobbing" in the story. On the one hand, Bernice gets her own hair bobbed when forced to show she is serious about it rather than just using the possibility of getting her hair bobbed as a line to make herself appear attractive. However, on the other hand, the title could equally refer to the second incident of bobbing in the story, which is when Bernice gets her revenge before she leaves her cousin by scalping her at night while she sleeps. The "her" in this case would therefore refer to Marjorie, as Bernice, having been forced to bob her own hair by her cousin, is more than eager to get her revenge by bobbing her cousin's hair in exchange.
The title therefore both foreshadows and hides the conclusion of the story from the reader in a very clever way. Before reading the story, we expect it to be about nothing more than Bernice bobbing her own hair. We are therefore surprised and shocked (and perhaps rather pleased?) when we realise that the title also pointed out that Marjorie's hair was also going to be bobbed. It was her, after all, who told Bernice to use the line that gave her such popularity.