I know I'm late, but I wanted to throw my 2 cents in. From my experience in MUSHing primarily in WoD spaces there typically isn't a lot of representation, but that's usually due to locale. Especially those set in New England and the Pacific Northwest. However, when I see games set in places with large black and other POC populations it always struck me as weird that every NPC was white. It's not a crime nor do I think there's even a hint of anything malicious, but it always looked weirdly, I guess, colorblind. I do have to admit that the majority of my experience with MUSHes are in WoD until relatively recently.
In the non-WoD and rostered games I've tried there's rarely non-white options on rosters or regions. I can't speak for Arx if anyone wants to bring them up as I only poked at the place for a few weeks before dropping off the planet. The more bothersome bit is what I see in fantasy games regardless of the medium. Essentially shoving Arab and Sub-Saharan cultures together, and calling it a day. I guess it bothers me because I don't see it as much for other locales and cultures. An easy example of this, if you don't mind the tangent, are the Redguards in The Elder Scrolls series. I won't lie. I'll take that over black folks not existing in a universe more often than not.
What @HelloProject was talking about is also something I've seen and known quite a bit in role playing when it comes to POC playing white characters. Before my time in MUSHing and in java/iirc I would sometimes get shit for not playing a white character in a predominately white area (seriously, something is up with WoD players and a love of New England), not typing out AAV (even if that's a large part of how I talk ooc), or, my favorite, not understanding why I really don't care for the vast majority historical settings.
In case it does need to be said: I've never felt attacked or like my presence wasn't welcomed on a MUSH due to my skin color or that of my PCs. I'm black, the vast majority of my PCs (I know I have had too many), and the only times anything troubling has ever come up is the typical "why would someone of this ethnicity be here!" or some of the typical exoticism. I also don't fault or think there's anything wrong with white folks wanting to stick to playing white PCs because it's what they're most comfortable with. On the flip side, I don't think there's anything wrong with those that wish to play a POC. I just think that, as HelloProject said, you need to learn to hold that L if someone points out that your portrayal is backwards, fucked up, or just outright ignorant.