I think it’s because it was seen in a lot of popular stories and then applied to less popular stories and so on and eventually became somewhat of the “norm” a bit like how a lot of mafia stories on the app have the same or similar plot lines, but there’s the fact that there were very popular movies that had such tropes (as the previous poster said) and they could’ve set the benchmark of such stereotypes.
Mean girls are notoriously portrayed as blondes and people are used to it, it’s probably likely a blonde is the first image that pops into their minds when a “mean girl” is mentioned. ‘Blonde’ is also regarded as a popular type liked by many men, I guess that can contribute to most mean girls in stories being attractive/somewhat attractive, especially to male peers despite their personalities (also rare for anyone to make the mean girl ugly by social standards). She’d also lose some or a lot of her power of MC (likely her influence too on other peers) if she was not portrayed as physically beautiful. The flaws of the mean girl become the MC’s biggest strengths, so to me, in a way she also acts as a comparison of why MC is “better.”
The mean girl could be viewed as somewhat of a stereotype template, the character of the blonde mean girl is already widely seen, recognized, predictable and accepted as apart of a college or high school plot.
As for gay best friends… I think the same applies, except in most cases it’s just a false sense of diversity. But the gay best friend is another template-like stereotype – predictable, recognized and (sometimes) accepted as part of the college/school plot, hence an easy add-in that requires little to no character development and effort.