I don't see the skulls as being childish or childlike, but that's just me personally. I don't really ascribe skulls in particular to any age group. Cartoon characters? Sure. Hello Kitty? That too. Not skulls. If you want to put a lot of thought into the skulls, think of what they commonly symbolize: death. That seems like a perfectly mature subject to me.
http://en.wikipedia.org/wiki/Human_skull_symbolism
Again, I think she maybe just happens to like the way they look. I think they look cool too, especially the ones DrSmith linked - sugar skulls from the Dia de los Muertos. As others have pointed out, skulls appear on lots of fashionable items made for grown women like
Alexander McQueen scarves, and the bikini in question with the skull on it. I guess that bikini might make for a funny skull-shaped tan line, but I think it's really cool.
I really don't see any reason to see the skulls as immature, tasteless or silly. Besides, it's her personal sense of style... it seems really wrong to me that we are trying to dictate what she wears and whether she gets tattoos.
She does not belong to us and as a grown women she can wear what she wants. She doesn't owe us anything just because she puts herself out there as a musician for us to listen to, either. I'm sure you guys would be really sad if someone tried to tell you what to wear and called your sense of style a silly phase.