It occurs to me that, before I launch into a discussion like this, I should probably clarify that I am specifically talking about the novel canon here. I cannot speak for donghua/cql Wwx, as I think they're subtly different in important ways, and I'm not sure which versions of the story y'all are coming from.
But with that said, I mean, @k1201a, you're absolutely right that Wwx's self esteem is high enough that he doesn't need other people to bolster it. Like I said, he knows that he's amazing! And honestly, I went back and took another look at the nightless city and the second siege, and he does actually stick up for himself and call out others' errors and hypocrisy with regards to him more often than I originally remembered him doing. So you're right about that, too.
I do still think I have a point about his arc in the present day being about not accepting unjust treatment, though!
I think what I should have said, rather than that he doesn't fight back against mistreatment, is that he always seems to take others' mistreating him as a foregone conclusion. Like, when he's confronted by others and accused of things he didn't do, or when he's treated as a villain for behavior that others could get away with, he always points out these things to whoever he's arguing with. But his arguments always come with this sort of cold, jaded tone to them, like he was expecting this all along.
You can see it clearest when he's confronted by somebody that actually does want to help, rather than assume the worst of him, and he's unable to accept that as a possibility. When Lan Zhan and Jin Zixuan try (admittedly in less than perfect ways) to help him in the nightless city and Qiongqi pass, he's unable to see it in part because he can't believe that someone from the cultivation world wouldn't be working against and villainizing him.
And even all the times that he's right about the fact that people are going to villainize and scapegoat him for their problems, that doesn't mean that "everyone's going to assume the worst of me" is a healthy default assumption to take to the world.
You're also absolutely right that one of the big themes of the book is the failings of the mob mentality and the ways that cultivator society exploits the weak. People resent Wei Wuxian and the Wen remnants and use his saving them as an excuse to vent that hate. In addition to that, though, I think that post-resurrection, Wei Wuxian's character arc (the biggest way he changes as a person of the course of the story), is about what kind of treatment he expects.
He starts, immediately post resurrection, shocked that Mo Ziyuan isn't scared of him, as he's used to being an object of fear. And when Lan Zhan first brings him back to the cloud recesses, he can't believe it's not because he's being arrested. He's still assuming that everyone is going to hate and assume the worst of him. But over the course of the present storyline, he discovers that Lan Zhan has never thought the worst of him, and he eventually comes to rely on having someone at his side who believes in him.
Because the thing is, like you said, Wei Wuxian doesn't need others to think well of him. He can get by perfectly well on his own (and do a lot of good while he's at it), but that doesn't mean that being alone and hated is a pleasant way to live your life. That doesn't mean that Wwx, even if he claims not to care what anyone thinks, isn't thrilled to find somebody that thinks he's incredible.
I think that one of the most important lines in the novel, in terms of Wwx as a character, is the one we get when he jumps from the tree into Lan Zhan's arms. "He wasn't scared of falling. All these years, he'd fallen many times. But falling on the ground still hurt, after all. If someone was there to catch him, it'd be more than wonderful."
He's finally affirming for himself that Lan Zhan really is there by his side believing in and helping him, no matter what, and now he can come to rely on that. He may not need the help, but he spends the present arc discovering that he can have it anyway, and he doesn't have to assume that he'll be misunderstood and mistreated.
And @suyanzhi, I actually largely agree with you. You're right in that I did initially get it wrong when I said Wwx doesn't stand up for himself enough, because he really does. The important part, though, for the point I was making, is that even though he corrects people when they're wrong about him, he never seems to actually expect anything different.
And as for Madame Yu, I absolutely agree that Wei Wuxian didn't have a choice about not fighting back against her. It only would have provoked her more. You really cannot expect a child/teen experiencing long term abuse by an authority figure to stand up against that authority. My original point was, in fact, largely that it's this original mistreatment by Madame Yu which sets up his outlook for later in life. It's the expected, forgone conclusion that when something goes wrong in the Jiang clan, be it anything from the disciples making trouble in town to the Wen clan wiping them out, Wei Wuxian is going to be blamed. Madame Yu does it, and Jiang Cheng, because he is also a child under Yu Ziyuan's influence, sometimes does the same.
The problem is that, though, is that he carries this sentiment with him as an assumption when he grows up. "I should assume I'll be unjustly treated as a bad actor" is a pretty realistic mindset for his time in the Jiang clan, but it's not a happy or healthy way to see the world. And when he gets more and more desperate toward the end of his first life, and when more and more of the world really does turn against him, he becomes unable to see when someone like Lan Zhan really doesn't think that way.
So to circle back a bit to my original point, I do think what I said still stands. Wei Wuxian is someone with very high self esteem, but absolutely no expectation of good treatment. He might argue with the people who mistreat him, might point out that how they treat him is hypocritical and wrong, but he never actually expects any different.
He doesn't really expect anyone else to see how amazing he is, and a big part of his journey is finding how happy it makes him when someone can see it.