Cosplay (e.g. dressing up as anime characters), goth and various other less mainstream fashion hides how naturally beautiful (by our current cultural standards) you are or aren't. Costumes cover you more and divert more attention to your clothes and accessories, and they sometimes include heavy makeup or even a mask. This is attractive to people who'd have less success with mainstream beauty. It's not a coincidence that you get lots of less-pretty people in fringe groups favoring appearance styles which make their physical appearance less relevant. Less-pretty people also decide to be trans at higher rates.
Cosplay also lets you put lots of effort into your appearance. It makes it socially acceptable to try hard. You don't have to pretend, "This old thing? I just threw something on." when you make a costume. That's appealing to people who are less naturally beautiful (by our culture's current standards) and want to put in effort to compensate.
More mainstream fashion has a lot of behind-the-scenes effort where you do subtle makeup and try to look like you didn't put much effort into your appearance (that would be shallow!).
This relates to the "law of least effort" explained by PUA site Girls Chase. The law is clearly explained in the book: whoever appears to put less effort into a social interaction has the social status advantage. Note it's about appearances, not actual effort behind the scenes.
Cosplay is an exception which lets you openly put lots of effort into your appearance in a socially-acceptable way.
So what? I think social dynamics that no one talks about or points out (afaik) are interesting. And mean.
What's mean about the social dynamics of cosplay?
I know you think social dynamics in general contain tons of meanness. Did you have something more specific than that in mind?
pressuring people to be pretty is mean
pressuring people not to put much effort into being pretty, even though they are supposed to be pretty, is mean
pressuring people to hide the effort they put into being pretty is mean
pressuring people to be pretty in ways controlled by their genes is mean (e.g. some skin colors and breast sizes are prettier than others)
cosplay and goth are partially strategies which help cope with a fraction of this meanness