I keep seeing anti-Maiko people complain that Mai never had a redemption arc to learn about why the Fire Nation was bad, and that's why she shouldn't be with Zuko. And I think that fundamentally misunderstands what a secondary character is. Mai is not a main character. She is a secondary character. She is there to provide support to the narrative, she is there to fulfill her plot purposes and that's it. That doesn't mean she has no character development, but her character development is mostly related to her function in the narrative. In Mai's case, her main functions in the narrative are being Zuko's love interest, and being a key piece in Azula's downfall. Her character development is related to that: we have a girl who is closed off and suppresses her emotions who gradually starts to open up a little through her relationship with Zuko. We also have her feeling like she's always trapped by fear and by others expectations (by her parents and by Azula), and because of her relationship with Zuko, she is able to finally stand up to Azula. That's her character arc. That's what's relevant to her function in the story. Learning about how the Fire Nation is bad is not relevant to her main plot purposes in the story. There's no reason why it can't happen offscreen after the war, with Zuko teaching her about all the things he saw and learned about the Fire Nation during his travels. She is not a main character, the show doesn't need to focus on every single thing about her character. In case people forget, we don't get a "redemption arc" for Ty Lee either, because this is not important, neither Mai or Ty Lee are meant to be the focus of the narrative, we don't need to see them "learning that the Fire Nation is bad".
Another thing that people don't understand is that not every character has to be a copy of Zuko. People act like the only acceptable way for Mai to learn about the wrongs of the Fire Nation is by having a redemption arc like Zuko, but that's not really the case. It's not even feasible: Mai didn't go through the same experiences as Zuko, she wasn't forced to live as a commoner amongst the Earth Kingdom peasants, she wasn't forced to flee as a refugee, so she never really had the opportunity to speak to these people and see their plight the same way Zuko had. Of course Zuko is the one who gets the redemption arc, because he is the main character for which this arc actually matters to the story, but also because the circumstances of his story allow him to have this arc, while the circumstances of Mai's story don't. But that in no way means she is irredeemable or that she can't learn, it just means she will learn about the Fire Nation in a different way than Zuko did, probably through Zuko himself telling her about his experiences and the things he saw. And there's nothing wrong with that! Mai is not a main character, she doesn't need to have some epic redemption journey. She was a Fire Nation noble, just like Zuko, she was taught the same kind of propaganda that Zuko was, so of course she's going to believe in it and believe that Zuko is betraying his country when he tells her he joined the Avatar. But Mai is also not some cruel person, and she's also not a Fire Nation fanatic. If after the show Zuko told her about all the things he saw, she would listen. And in fact, the evidence we have is that she did listen to Zuko and she did learn about the Fire Nation: in the comics she works with Zuko against the New Ozai Society even after they break up. And if you don't want to use the comics as evidence, we have the epilogue of the show as evidence, in which Mai is shown to be on good terms with Team Avatar and playing Pai Sho with Suki. We don't need to see the details of how Mai learned about the Fire Nation being bad (she is not a main character, it's not relevant to the main story), we know that she did learn, we can fill in the blanks with our imagination.
Secondary characters not receiving a huge narrative focus and character development about every single aspect of their characters is not "bad writing". Bad writing would be if the show focused on every detail of every secondary character, because that would be losing narrative focus. Just because certain people don't like Mai with Zuko and use this as an excuse to say why the ship is bad, it doesn't mean that Mai not having a redemption arc is "bad writing".
Finally, this isn't a reason for her not to be with Zuko. Some people argue that Zuko would never want to be with Mai after his redemption because of her still believing that the Fire Nation was in the right, but people forget that if there's one person who knows what is like to have been brainwashed since childhood with propaganda about the Fire Nation, that person is Zuko. And Zuko was way more enthusiastic about the Fire Nation being right and the Fire Nation winning the war than Mai ever was. Why would he hold it against her that she believed in Fire Nation propaganda, when he himself knows what is like to believe in said propaganda? The most likely thing to happen is that he would want to teach her about everything he learned about the Fire Nation, he wouldn't break up with her just because she didn't magically unlearn an entire life of indoctrination.
I love art improvement. It's like one moment, you draw hands as blobs because they're too hard and you're never going to master it. The next (years later), you draw an excellent hand and realise maybe all you needed was more time.
Why is Zuko romantically interested in Mai? What does he like about her as a person? Does he understand her personal desires and struggles?
"Why is Zuko romantically interested in Mai?"
He's a moody, angsty, grumpy teenage boy, she's a moody, angsty, grumpy teenage girl. She hates everything, but she doesn't hate him, and he appreciates that. He practically brags to a guard at the boiling rock that his (ex) girlfriend doesn't need anyone to protect her because she can handle herself.
They find each other physically attractive, enjoy making out and bitching about things they don't like, and they genuinely have a lot in common (troubled family life, struggling with the idea of who they should be VS who they actually are, don't like it when people try to control them, want a comfortable life where they are free to do whatever they want, and experts at using cool weapons that they're not allowed to kill people with because it's a kid's show).
Also Mai knows how to be understanding, caring and supportive without babying him, something Zuko desperately needs and actively seems to want in his life. And while her "aloof", more detached personality sometimes clashes with his need to shout about every single emotion he feels, it sometimes has the very healthy effect of making him chill a bit (and makes her come out of her shell a bit more).
"What does he like about her as a person?"
Like I said, he likes that she genuinely cares for him, but doesn't put with his bullshit, AND he likes that's just as much as an angsty teen as he is. He literally makes a move on her when she finally had enough with their friend group and yells at everyone, including him, to leave her alone. He looks like a lovesick fool when thinking back to "that gloomy girl that sighs a lot." Boy literally said "You're so beautiful when you hate the world" and was being 1000% sincere.
He likes how bold she can be, how she doesn't put on an act for him and how he doesn't have to put on an act for her, and how she literally risked it all to save him and is willing to be by his side now that he's about to start a new era for their nation even though she didn't fully understand why he changed sides.
Mai is, before anything, her own damn self and won't let anyone dictate her life for her, and Zuko really admires her for it because he can relate to that - which leads us to:
"Does he understand her personal desires and struggles?"
Does Zuko, the banished prince that rebelled against his abusive father, his nation's cruel and violent ways, and even against the father figure that he loved dearly but that wasn't listening when he said he wanted more in life than just a job and a roof over his head, understand what's like to only get conditional affection, be told that he needs to shut up and let himself be bossed around if he wants literally anything in life/not to be harmed, to not be listened to even by well-meaning people that just don't get it, and eventually risking everything to carve his own path in life with people he believes actually vallue and understand him?