Mean Girls wasn't a documentary, and as a former teen girl, I'd argue that girls aren't actually uniquely awful: what they are is especially sensitive to social cues and influences. You may note that the article here isn't really about bullying, it's more about girls making themselves miserable by comparing their lives and their bodies to what they see online. I do actually think there may be an evolutionary element to the fact that females are (on average) more oriented to language and interpersonal skills, more attuned to social feedback, and on the flipside, more prone to anxiety as a result, but I think it's a mistake to view that through too simplistic a lens.