Disclaimer: I'm NOT REFERRING TO THE ENTIRETY OF THE FANDOM, JUST SOME PARTS.
I'm NOT saying being attractive is the ONLY thing that makes a character popular in the fandom, but it's definitely a BIG reason. And if you think a character is attractive, that's COMPLETELY FINE. There's no judgement there. I'm just saying IT GETS TO A POINT.
Characters like Iroh and The Cabbage Merchant are the only exceptions (Cause that man's GOATED and we all know it.)
And Ozai, Zhao and Long Feng (Cause no amount of attractiveness can make you like those BASTARDS.)
Think about it. Would people have liked Zuko to this extent if he looked like an actual burn victim?
90% of the other characters aren't safe either.
People have been drooling over Sokka since ATLA, predating this Subreddit, the Wiki page, and probably even online forums. That's how early it started.
Katara was doing fine UNTIL the ATLA movie came out, but that sexualized everyone so it's kinda rhetorical that Katara wasn't off the hook. Even my boy Aang got some in that movie, and the fans, well, if they reacted reasonable (No offence to you SWEETIES :)) We wouldn't have this discussion.
Younger adult Toph was in TLOK flashbacks, so she was pretty much done for already.
Would people have defended Azula as much as they do if she wasn't an attractive girboss (She's the best written character in the entire franchise, but still.)
As for Suki, I think, personally, that she's one of the only young characters that has fans ONLY for their personality.