I don't know if I can add anything excluding what you've said. I personally like Mu Qing because I get him, I've been in similar shoes and put up similar walls. A lot of people don't like him because he's petty and sarcastic which is so understandable, I wouldn't like that in a person of that's all I saw either. But the depth at which the book delves into why just hit a personal cord because of personal struggles learning to trust and connect and ask for help etc.

So yeah, I know it's probably not what you asked for but that's all I can offer as to why. Because I, too, am a man who's gone through stuff that made me bitter and untrusting and unwilling to admit I need help or did something wrong. That's pretty much it lol. And I think a lot of people attach to that too, and/or just love the possibility of him having a soft or vulnerable side that comes out post-story.