Aqua's back looks within reason to me, looking at arched_back, most of the posts have this level of bend.

"Poor quality." You keep using that phrase. I do not think it means what you think it means.

I would say it's in-line with ballet dancers. That pose isn't humanly impossible...let alone this is an illustration, take it easy? The only complaint I have here is the resolution...it's amazing if I were still using a 600x400 monitor, now this looks like a thumbnail in my 4k.

Barring the "arched back" controversy, i like how they actually show aqua being large breasted but nowhere near Darkness's bustyness. It seems so off making aqua and darkness the same in chest size in so many other arts.