It's all because of the image Nintendo wants to have, I guess. Yeah there are some games like devils third that are mature, but nudity is not seen as good as killing people in a videogame, as absurd as it may sound.
I wouldn't mind having the option to play as a half naked woman in a horror game. Have you ever seen a horror movie before? How many half naked girls with big boobs are there? It might be something common in that genre, right? I don't see the scandal in any of this. And no, I don't think people would look at those as something sexy.
Come on, we all have smartphones and internet, porn is so easy to find nowadays, I don't think those costumes have anything to do with porn xD