I don't think it's just the target demographic. I think this desire for self-expression was always there, but game producers/developers have only recently fully embraced the idea.
I have gamer friends (~30 years old) who joke about "Fashion Destiny" (referring to the video game series) and so on, except that they clearly really do care about the cosmetics because the availability of certain cosmetic pieces will get them to go back into the game and grind out 50 hours of gameplay, or cough up $X for Overwatch loot boxes, or pre-order, or buy the battle pass, etc.
It never gets to the point of bullying because we're 30 years old and we all carry guilt over the frivolity of dumping money into virtual cosmetics. But if you want a bunch of 30 year old men to talk about accessorizing like it's Sex and the City, just throw us into an in-game winter festival with cosmetic loot boxes.
With Fortnite, I think there's a problem where the winner to loser ratio is so small in each round that win/loss and other performance metrics become meaningless, so there isn't some sort of reckoning in the form of the end-game scoreboard. With games I used to play (as opposed to the ones I play now), the shit talking mostly got settled when the other person outplayed you. Or vice versa.
On the one hand I think the Fortnite dynamic helps to keep the game accessible, but on the other hand for lack of a clear direction (getting good at the game) players get a bit more cosmetic.