Well, to me it looks as though her butt is covered, as least as much as a standard athletic swimsuit would cover it, and not the kind of wedgie-thong you see on a lot of characters.
But yeah, it does look a little weird, though I think it's the hip/tosro area that's off, and that may be due to the fact that it's all black and in the distance, so it kind of mutes any differentiation.