I don't see that as being nudity, but I could see how the mod would call it mature. It seems to me it could go both ways.
Ultimately, its the mod's decision. It up to you where to go from there. You can accept it and move on or try a different group. I've had pieces turned down by groups before too. Some decisions I accepted, but a couple groups I left after several rejections, because I realized we weren't going to see eye to eye.
I can see why they categorised it as nudity, regardless of how "there are some things stuck to her body" may not make her literally naked.
1. Her veils are transparent, and it seems like you would be able to see the nipples and genitalia had you chosen to draw them in a more detailed style. I can see her blank crotch through her veil, which means her veil is not covering her nudity... If someone has no genitalia, is it impossible for them to be naked? Of course not.
2: Her outfit is clearly not designed for everyday purposes, it looks like she is a participant in some sort of burlesque show. There is no reason for her garments to be so scanty other than to display her nudity as much as possible. So you can't really complain when people notice that.
It strikes me as the sort of thing people do in a deliberate attempt to sneak nudity past censors. Not that I have any paticular problem with nudity, burlesque, or pinups. I'm all for these things. Just call a spade a spade.
I also actually like the character design, but I think you should make a decision one way or the other about whether she's going to be naked or not. The stick-on veils look a bit odd, like an awkward half-measure. If it's not important for her to be naked then you could easily rig her up a more substantial set of lingerie without detracting from the design.