My theory is relatively simple: skins/costumes are signals and signals are important. Fortnite has matchmaking, and the majority of players use the matchmaking function in the two-/four-player modes rather than with a premade group. The average player you are matched with will be Fortnite-matchmaking replacement level[1]. If that player doesn't have a skin, they are more likely to underperform, from your perspective, since the pool of "non-skinned" players also includes new players and marginally-invested players (i.e. players who rarely play). I think a lot of comparisons to fashion are trying to solve the symptom, rather than this deeper problem.

Youth sports has a similar phenomenon. If you show up to the first day of practice with "default" athletic gear (running shoes instead of cleats, cargo shorts instead of athletic shorts etc.) you would face a similar type of shunning. The reason why there isn't a push to ban expensive athletic equipment is due to 1) The equipment manufacturers have huge sway over youth sports organizations via sponsorship 2) kids like athletic equipment because its one of the few outlets of individual identity available to them. That's why you are more likely to see displays of individual or group altruism (e.g. what happened to your son) rather than questioning the ethics/fairness of this phenomenon.

That is also why I prefer Apex Legends' freemium model to Fortnite's. Yes, it does include loot boxes (which are intrinsically exploitative), but it also includes ways to signal gaming ability without paying money, such as opt-in game statistics (how many wins with a character, how many kills, etc). More explicit signals of competency make weaker secondary signals (having the right skin/kit/cleats) a lot less relevant, even counterproductive (being labelled a poser).

[1]https://www.baseball-reference.com/about/war_explained.shtml