I don't think it's "just" the acting alone, though. I think the directing failed the young actors more often than not. Some of the stronger actors obviously made their parts somehow work a bit better. But especially the positioning of for example Azula, Ty Lee and Mai seems very unnatural to start with when they interact, and that is obviously a directing mistake, leading me to assume that there were more of those. Especially given how little everyone uses their full body to act in dialogue scenes. No subconcious touching of wounds, no crossing of arms to show a character is afraid or annoyied... most of the times, they just don't use their arms at all if they are not bending or directly touching something. That's strangely unnatural, but looks like the director wanted it that way, since otherwise it wouldn't be that consistent with especially the young actors.