Jessica’s answer to “can I just ask why this isn't marked as a childrens book anywhere because it was sold everywhere wi…” > Likes and Comments

Me, I think a retelling that affects the reader the way the originals would have affected the hearer is a Good Thing.

If by "a solid look into the mythology" you mean more than a brief acknowledgment that Gaiman has combined different sources into a single story (cf. modern Bible criticism), well, no. But that wouldn't make it a more adult book, just a more academic one.