Let me tell you about Homestuck
A month ago, the webcomic/cartoon/game/mixed-media-absurdity phenomenon Homestuck came to its conclusion in an oddly fitting way: Homestuck became an anime.
Well, not really. What actually happened was that Act 7, the series' final act, was released on April 13th, 2016, seven years after the series started, as a 9-minute animation. And unlike other animations in Homestuck - which typically featured very limited animation drawn in the comic's simplified art style - this one had a more complicated anime-inspired style. In addition to being fun to watch, this also served as an in-joke, as it had been a meme for years to call Homestuck an "anime." This wasn't like Avatar: The Last Airbender, where people who didn't know better would look at its anime-inspired style and confuse it for anime, or like Corey in the House, where it's jokingly referred to as an anime by its devoted (and wholly ironic) online fan base. No, Homestuck got jokingly called "anime" for a different reason:
For a few years, it was EVERYWHERE at anime conventions.
Wait, aren't these anime conventions?
If you go to any anime convention, you'll see a large percentage of cosplayers are dressed up as characters from non-anime and non-Japanese sources. A few years ago Homestuck and My Little Pony: Friendship is Magic were the big popular trends for Western media cosplay, now it's Undertale and Steven Universe. Finn from Adventure Time has surpassed L from Death Note as the go-to "slap on a T-shirt and wig/hat" low-effort cosplay. There's usually a good number of Doctor Who and Avatar/Korra cosplays, and Disney movies are always popular sources for cosplay (and yes, some of those can claim to technically be manga characters because of Kingdom Hearts). And there's always at least one Deadpool acting in-character.
Why so many non-anime cosplays at anime-themed conventions? Well, conventions are one of the few places were cosplay is socially acceptable, so anime cons offer an opportunity for cosplay of all sorts. Often it can be a way to further bond with your fellow fans; it's assumed everyone at an anime convention is going to like anime, so if you meet someone who's cosplaying, say, Furiosa or Walter White, you have something else to talk about with them in addition to anime. There's also a social dynamic by which anime cons, especially if they're in an area with few general media conventions, can attract people with only a passing interest in anime to attend for other fandoms.
So it's OK to cosplay things other than anime at an anime convention?
It's absolutely OK! I've done it multiple times myself! There's lots of great characters you can cosplay!
That said, there can be ways people in non-anime fandoms can behave intrusively at conventions. I remember back at Anime Boston 2012, the year the con was overflowing with Homestuck cosplayers, there being issues with the giant Homestuck meet-ups blocking hallways, which wasn't fun for anyone. Of course, anime fandoms can be equally obnoxious. In some ways more annoying were the more entitled fans who whined on the convention forums about Anime Boston not hosting Homestuck-themed panels and claimed the con was "discriminating" against them. Now, some vaguely anime-themed conventions do run some non-anime programming to attract wider crowds, but Anime Boston's specific non-profit mission statement requires they focus their official programming on Japanese culture, so just giving panels to Homestuck or Steven Universe or whatever else was popular, without a specific Japanese culture connection, could actually threaten the con's non-profit status.
As such, I welcome non-anime cosplayers to participate in anime conventions, but on the condition that they're not trying to take over the convention as their own.