I think the only thing missing from this, is room for cultural appreciation. As you state, cultural appropriation is taking aspects of an oppressed culture without understanding the whole culture. The way to fix that, is to make genuine efforts to learn about that culture and find ways to express appreciation with sensitivity.
Of course, no culture is monolithic -- you will always have members who may find offense and others who do not, making it tricky. But I think most of the time when there is an honest effort to learn and engage, not just hijack the fun parts, people are appreciative of a dominant cultures desire to learn.
I'm not sure where this costume falls on that spectrum, just speaking generally. The costume is part of a movie franchise, so I think the real question is, did Disney do a good job of telling the story of this Polynesian myth? And I don't know enough about either the film or that culture to have an opinion.
