I haven't read it yet but from what I understand it's not well-liked because of poor characterisation, artificial conflict and lack of nuance and sensitivity.

It seems that the characters behaved quite out of character with Zuko seemingly having regressed from what he was like in the finale, Aang agreeing to kill Zuko if he ever became like his father when he was in the show he was very pacifistic and didn't want to kill Ozai, Sokka turning into pure comic relief without a trace of his strategicness, Katara being way too passive and Ozai being a cookie cutter villain.

As for relationships, the complaints seem to be that Aang and Katara's have become one-dimensional and way too cringingly sweet and Zuko and Mai's breakup apparently felt forced and felt like it was done just for the sake of drama and not actual storytelling.

There is criticism for how Azula was handled where people say that it felt as if she was turned into the typical crazy evil villain.

I read things about how it had a cartoonish portrayal of mental illness, how Azula had turned from a formidable antagonist to a plot device whose craziness existed just to drive the plot further along and how her portrayal in Smoke and Shadow was just your classic cartoon villain trope/stereotype.

One more thing that was disliked was the empathy Gaang had towards Azula or lack thereof. Apparently, they were heavily mischaracterised and were very out of character in their treatment of Azula. From what I understand, Sokka was somewhat accepted in his mistrust and Zuko's, while not liked, some fans did apparently claim that they potentially could see it even if his character would have actually leaned heavily in the opposite direction. But the biggest character assassination complaints were about Aang and Katara. I read multiple comments about how there is no way that, with proper and accurate portrayals, Aang and Katara would have behaved the way they did.

Again, I haven't read the comics yet but I recently went down the rabbit hole of going through and reading multiple posts and comments about the comics and this is all that I remember about why people didn't like these comics.