I saw this movie in the original French version and agree that the environmental theme is "Miyazakian". But the backgrounds and character designs are certainly not - the painted backgrounds owe more to Cezanne or Van Gogh than Miyazaki's pastoral realism - and the human characters look nothing like anime characters. As for the Migoo monsters, these are much more cartoony than anything I have seen from Miyazaki.
This is a strikingly beautiful film which will appeal to all ages. It's so refreshing to view a film for children/families that's so graphically different than anything else I've seen. And the story holds together. My 6 year old son loved it and couldn't stop talking about how beautiful the film is, something he's never noticed in any film before.