I definitely don't think this drama is over exaggerated at all. Girls do be like that, especially high school girls.
Remember that in the previous timeline when Hoshigasaki got isolated, it was pretty early on, so they weren't really good friends then. I still think it's dumb for her to be judged and isolated for liking light novels, but again, high schoolers.
Now, they are much closer friends, so yes, Hiiragi is being a bitch. But as was pointed out, Hoshigasaki hasn't properly explained herself either. Really, she needs to tell Hiiragi the truth about Nanamura, which I don't think she's done, yet. That would clear pretty much everything up and also probably stop Hiiragi being jealous.
But again, high schoolers and drama.