I suppose it depends on your perspective. I have the deck...don't use it to read, much, but wouldn't part with it. There's something very interesting about it that makes me hold onto it...some quality or mystery that makes it feel very valuable. Perhaps mermaids have that effect
If your daughter loves mermaids, is mature, and you're both comfortable with it, I'd say go for it. She's much more likely to feel connected with the deck if it's presented in a form she enjoys. I personally don't find the nudity "graphic" at all--I think it's tasteful, I mean, they're mermaids. I think we just get caught up in the whole notion of what mythological creatures and fairytales are supposed to be, and what's appropriate for kids, since Disney came along. An overgeneralization, perhaps--but go with your gut, not with what's supposedly "appropriate."