I talk a lot about sexism in fiction and stuff, probably a lot more than you'd expect given how much I also talk about how superficial traits don't matter and leftists are often hypocrites about that. So I thought I'd explain my reasons for caring so much about the representation of superficial traits in fiction.

Short answer: a mind's instinct learns by association. Therefore, giving most or all characters one superficial trait when its negation is as common in real life, or repeatedly lumping a given superficial trait on villains tends to encourage double standards in real life, besides being insulting. (The reason it's insulting even to reasonable people is because the mind inherently works this way and so they'll notice it, although at least it won't poison their ideas.)

(Interesting anecdote: I heard some people talking about some statistic that a disproportionate number of criminals have one of three names, I don't remember for sure but I think two of them were Wayne and Lee. They even phrased it as "if you have one of these names, your odds of being a criminal are higher". I pointed out that that statement is necessarily false because obviously your name doesn't affect it from your perspective. At most it makes it more likely that someone else with one of those names is a criminal, but you already know yourself, so finding this out doesn't change the chance that oneself is or will be a criminal. I also pointed out that acting on this statistic creates a self-fulfilling prophecy: if we believe it's more likely that a man is guilty simply because he's named Wayne or Lee, then we'll end up convicting men with those names more often and therefore perpetuate the original statistic.)

So that's the reason for me valuing diversity in representation of superficial traits. If I seem to care a lot more about gender representation than about other traits, it's because I do. A notable difference betweeen gender and race is that racial diversity is not essential to survival in real life. You can have a society of people of only one ethnicity and that society can thrive, but you can't have a society of only men or only women.

I should also point out that diversity has to be considered locally. "You have no Chinese characters in your story with twenty characters!?! China is almost a fifth of the world's population, you know!" Yeah, well, not in America. A story made and set in America and expected for obvious logistical reasons to be primarily consumed in America should be held to the concept of diversity that reflects reality in America.

One other thing: lumping a superficial traits on villains all the time has a difference in kind from not portraying it at all. You can't excuse that with "it reflects reality". Besides the fact that such a claim is very difficult to establish reliably, even if it's true portraying it as false is definitely the better course of action for its effects on the culture and individuals of the real world. For example, it's a common claim among conservatives (and I find it believable based on the present culture) that black people in America commit a disproportionate number of crimes compared to white people. That doesn't mean we should portray this as the case in our stories. If I were an innocent black person and most of the black characters in stories were criminals, I'd be offended at being lumped in the same group as criminals just for my skin color and I'd probably form a bias against white people. The thinking would be, "white storytellers just assume people that look like me are criminals because some are, so I'm just going to assume white people I meet are racists until proven otherwise, because that seems to be statistically true". You can see how in this context trying to "reflect reality in stories" backfires.

The obvious exception, of course, is if the trait isn't actually superficial but something with undeniable moral signifiance, like religion. I won't once complain if all the Christians in a story are villains, because it's a pretty morally messed up religion. A morally messed up ethnicity, on the other hand, is a categorical impossibility.

Traits that aren't inherently moral but also aren't superficial, like skill set, should still be paired diversely with superficial traits in fiction even if it doesn't reflect reality. The point of stories is not to describe reality. It's to describe a possible reality. (Setting your story in "the real world" is a tool to avoid worldbuilding work when your story doesn't need an original world, not a reason to copy over even the most circumstantially variable aspects of it for no gain in story quality and no significant savings in work.) For example, if you're going to write a story about a group of programmers, it's still a good idea to use a close to equal gender ratio, even though in real life programming seems to be an almost entirely male-dominated profession, because such a trend in real life is due to cultural gender roles and not to a difference in biological fitness. Erasing those arbitrary roles is a good endeavor.

For one that probably is based on a legitimate difference in biological fitness, like physical combat, reflecting that in stories is fine. Just don't exaggerate it. It's not anything close to a hard rule in real life.