You can see that right before Keisuke overheard them, Hebikawa's group was teasing her about talking to the class loner, so she ended up calling him gross because she didn't want it to effect her status. In other words, I don't think she actually wanted to be mean, but she thinks that being mean is the only way to keep her image. It doesn't mean she's actually a pure girl at heart, but after seeing her backstory, I can sympathize with her.