Every flaw in the writing of Aa.ng's character can be explained by the fact that he is a self-insert and wish fulfillment fantasy for Bryke.
To be clear, I want to differentiate between a Character Flawā¢ļø and a flaw in the writing of a character. When I say flaws in Aa.ng's character, I don't mean things like his avoidance of his problems, his immaturity, or later on in the series his inability to regulate his darker emotions. These are Character Flawsā¢ļø, and every character needs them in order to be relatable, in order to feel real and not be boring. Aang having flaws is not a bad thing, it's just part of being A Character.
The flaws in the way his character is written, on the other hand, are the choices made behind the scenes by the creators that made him less likeable as the series went on. The Character Flaws he has are not things that condemn him to being labeled as a Bad Person who is worthy of hatred and derision, but it is the fact that his character flaws go almost entirely unaddressed by the end of the series that end up leaving a lot of people with a bad taste in their mouth about him by the end.
Do a close watch and take note of every time Aang does something wrong, every time he makes a mistake or harms one of his friends unintentionally, and then take note of whether or not his friends stay mad at him when they reasonably would and whether he ever apologizes to them before they inexplicably forgive him or seem to just forget what he said or did. There are definitely examples where he DOES apologize when it's needed, especially earlier in the series, but the times when he does not apologize vastly outweigh the times that he does, especially later on.
All of this can be chalked up to the fact that the things Aan.g would need to do to positively develop as a character are things that are not fun and therefore get in the way of the fantasy. Having your friends stay mad at you is not fun. Having to reflect on your actions and recognize your faults and be humbled and apologize for the ways you wronged your friends is not fun. Having to accept that the girl you've been pining over might not like you back and having to accept the possibility that you won't end up with her is not fun. Having her stay mad at you for yelling at her, for violating her, or for continuing to run away from your problems, is not fun. Having to apologize for all these things and admit your faults and grow from them is not fun.
What is fun is not having to ever be held accountable for anything, not having to give anything up in order to win, and getting the girl in the end anyway despite never making anything up to her.
I don't hate A.ang, but I'm censoring his name in this post because his stans tend to have fucking rabies and will foam at the mouth if you say anything critical of him or the way he's written or point out the ways he treats Katara poorly or imply that it's not a foregone conclusion that he NEEDS to end up with her.
He could have been a stronger character, a better written character, if they had actually let his flaws be flaws, but they don't. It's not that they don't give him flaws, it's that they don't LET the flaws be treated as flaws by the story. He doesn't have to grow from them.
Often when criticizing Aa.ng's character, instead of refuting our points, his stans will just endlessly deflect to Zuko's actions and try to claim that we hold Aa.ng to a way higher standard than Zuko because people who criticize Aa.ng tend to like Zuko more, which may seem hypocritical from the viewpoint of an A.ang stan, because Zuko does way worse things than Aa.ng. Zuko stans who criticize Aa.ng do not think that A.ang yelling at his friends and acting like a dick and then not apologizing for it is somehow worse than the villainous stuff Zuko did, like (however unintentionally) setting fire to Kyoshi Island and showing no care for the consequences of such a thing. I'll die on the hill that canonically it was reckless negligence and not intentional arson, but that doesn't change that he didn't care once the fires started and consequentially the action has the same end result.
Zuko ends up being a more likeable character to us in spite of all of his crimes because he demonstrates visible remorse and growth in a way that A.ang does not. It's not that Aan.g does anything worse than Zuko or that Zuko's crimes are somehow more forgivable, it's that he takes steps to actually earn forgiveness in a way that A.ang is not made to by the narrative before being rewarded. Regardless of how much more mild Aa.ng's "crimes" are, comparing him to a character who has done worse things is not a replacement for actual growth.
A.ang's character declines in his likeability because his wrongs are not meaningfully addressed, he's just allowed over and over again by the story to get away with them and not have to atone or change for his friends to forgive him and pretend like nothing happened.
Zuko has to over and over again be humiliated and punished by the narrative, until eventually he is for the first time in the story rewarded for doing something heinous with everything that he thinks he wants and has to decide on his own, with no tangible reward waiting for him on the other side, that he's made a horrible mistake and must make things right no matter how ego-bruising it will be. Admitting you were wrong hurts, and the more wrong you were, the harder it is. And Zuko did A LOT wrong, so him having to admit that both to himself and to the people he's hurt and to try to make it up to them is a Herculean task for most people. The fact that he takes on that task anyway, and that the audience already understands that he is a character with deeply rooted self-worth issues, is impressive and admirable. It would have been way easier for him to do nothing, to keep living his empty vapid frivolous life of luxury and never have to face the people he's harmed, but he doesn't.
Comparatively, the fact that A.ang wrongdoings are much milder than Zuko's does not translate into him being the more likeable character, but the opposite. Because comparatively, it should be way easier for Aa.ng to admit his faults and apologize than it is for Zuko to do the same. He never burned down anyone's village, he never attacked anyone while their back was turned, he never threatened a bunch of terrified women and children with fire. It should be the easiest thing in the world to give him a moment of growth where he says something along the lines of "I'm sorry for how I spoke to you back there, I'm going through a lot but that doesn't mean it's okay for me to take it out on you." That is way easier to do than trying to prove yourself to people who you hunted and attacked across the world. Zuko's trauma and the fact he's had a rough life is never used as a replacement for his atonement. He still has to prove himself to them regardless of how he ended up the way he is.
A.ang's lack of character development can be directly attributed to his status as a self-insert for Bryke. Going through the growing pains associated with becoming a better person and developing as a character is not fun, so Aa.ng does not do it, because that interferes with the little-boy power fantasy they're trying to create.
And the most frustrating thing is that it didn't have to be this way, Aa.ng could have been a way better written much more likeable, relatable character if they had let his flaws be flaws and made him work through them, if they had shown us his growth. But they don't.
