On the movie and why the casting is the least of the problems
Shyamalan has been widely and rightfully criticized for whitewashing the ATLA main cast. Sadly said whitewashing is only the tiny tiny tip on a huge iceberg of racism hidden in the depth of a bad movie.
I can take only so much of that movie without wanting to throw something at our TV, so I will do this in bits and pieces.
Let’s start with the opening sequence. Notice something? All the Chinese characters from the animated series have been replaced with gibberish. Deliberately replaced with gibberish. There was no need to change them, especially since the rest of the opening was kept exactly the same.
This would be offensive in and off itself, but in the grander scheme of what Shyamalan did to the ATLA universe, it is the first step in his grand effort to remove anything Chinese from ATLA and replace it with India (in case of the Fire Nation, which he clearly seems to fanboy) or, when not possible to replace, portray as squalor and poverty and full of dumb people.
And there was absolutely no reason to do it. I honestly would love to hear the thought process that they had for that. “Oh look, the characters are already there, I know, let’s change them to squiggles! No-one will know the difference! Why? Do we get more XP for being racist?”