What, you're not familiar with the Japanese Lolita fashion style and its variations, as worn by countless thousands of legal adults in major cities everywhere??? The young women I know who've adopted that style would crack up at the idea they're attempting to imitate children.
People need to think beyond what they see - for instance, I'm clearly very short, but a quick glimpse at my profile reveals that I'm an Ancient Dark Elf with a literary bent. (I'm also pretty long in the tooth in FL, but that's incidental.)
Besides, there's nothing new about this kind of fashion - the first babydoll nightwear came into being in the 1930s, for goodness' sake.