People about Cosplay...
I’m going to address this question from a different viewpoint. Why is cosplay popular amongst those that do it?
There is the factor of honoring the source material/fandom that you love. There is the creative outlet. Then there is the sharing of the work with other fans and making new connections with like-minded people. Getting complimented for your work can create a sense of accomplishment, as can having a photographer ask to take your picture. A three day event like a local comic-con or Anime/SF/Fantasy Convention can be more rewarding than a two week vacation in the tropics, to some people.
This all has a spillover effect, those that aren’t doing it, see the results and fun that those that are having and want to do it themselves next time. This keeps the world of Cosplayers growing at a currently staggering amount. I’m staff on a 2,200 person Anime Convention and we are constantly amazed at the ~60% cosplay attendance.
I think it is just fashion modeling in disguise. (pun intended)
Overall it is a harmless fun. As a creator (artist or writer) there is no greater honor than someone loving your show so much they would dress as your character and even try to be like them. Even against the possible ridicule.
Playing pretend can be fun. And it’s something everyone even families can engage.
But if you look at the most famous and successful cosplayers (as in Twitter famous) they are either:
A) People that could be modeling in mainstream modeling agencies.
This one is the category that gives more envy and disappointment to your average cosplayers. Of course they will get more attention even if your costume is more “comic/book” accurate or well done.
Like I said, cosplaying is basically fashion modeling. They look amazing and it’s nice to see models having a nerdy side.
B) People with awesome disguises that resemble the original show or movie.
In this category you’re judged by your amazing sewing skills.
So for being ultra famous you need to be one of those categories. But if not it’s just harmless fun you do with friends.
Think of it as early Halloween.
Sadly as a lawyer it isn’t professional for me to do this. But I respect people that do.
When judging whether something is cultural appropriation, ask first: is there a cultural double standard that praises the borrower while punishing those whose behaviors/styles are being borrowed from? If not, then the second test is to see if the thing being borrowed has religious/spiritual/historical significance the borrowers are disregarding or disrespecting for the sake of ‘cool’. Finally, does the act of borrowing reinforce stereotypes?
To answer your question, No. Cosplaying in the past hasn’t been used to marginalize, dehumanize, or stereotype the Japanese people. Cosplay has no major religious, political, or spiritual significance, even in Japan it is considered a commercial hobby. Cultural Appropriation is problematic because the hegemonic group (in the US case, white people) get praise for behaviors the group that they borrowed the practice from get penalized for (in this case, Japanese otaku). However, Japanese otaku have the same or superior status as American Cosplayers at US Conventions and English language internet. Japanese Otaku are not facing a double standard when compared to white otaku that favors White otaku.
There are some racists who may say white cosplayers look more like the anime/video game characters then Japanese cosplayer, but those types tend to be seen as racists and pariahs in most well-balanced cosplay communities. Most people tend to think the Japanese have very high quality cosplay.