Knowing very little about the Polynesians, discovering they have a badass tattooed demigod in their history makes me interested in reading about their culture.
If kids who see this movie and are likewise inspired by the concept, inspired enough to DRESS like the character no less, how is that anything other than a positive? It's showing off an awesome side of a culture that rarely receives mainstream attention.
It seems to me that there is a very fine line between genuinely malicious cultural appropriation and the inclusivity where these neglected peoples are finally given a time to shine. Sending a message to children saying "you can only enjoy your own ethnic culture - no one else's!" seems FAR more racist and divisive than saying "wow, it's great you're showing interest in the perspective and background of other tribes and groups within our interesting and diverse species!"
With every new overblown news story of this nature, it seems like more and more people are passing over the rubicon and going SO FAR down the route of "this shouldn't be allowed because it's offensive!" that it's hurting their cause of equality and open-mindedness way, way more than it's helping.
I had a moment like this recently when talking to a European friend of mine. She said something about her "chocolate friend", and I asked what she meant. Turns out she was referencing a Jamaican pal she had, who she calls her "chocolate friend" because of her skin color.
My immediate response was "wow, that seems super racist!", but her explanation made me realize that "this is probably racist" is now my default mindset on a lot of these issues when I see them. There's just a lot of cultural conditioning setting the expectations that any discussion on the differences between skin color is inherently negative, so that's the knee-jerk reaction I usually get before I slow down and try to apply logic.
When I asked why she called her friend chocolate, she said "Because her skin is so soft and dark and full, just like my favorite sweet of chocolate!" I then went on to find out that's the skin tone she's finds most attractive, too. For her, it was a term of the utmost endearment, celebrating the differences and diversity instead of thinking it's some Voldemort-esque taboo that shouldn't even be mentioned aloud.
This feels similar. Lots of kids are going to see this movie and think the Polynesian peoples have a cool look to them. Why not let them immerse themselves in that, instead of preaching that it's only okay to look at other cultures from a healthy distance and that it's STRICTLY FORBIDDEN to take part in the traditions of a different group because it's a group you don't belong to? The latter is essentially saying "you should be excluded from this activity because of your race" (as I highly doubt those pissed about this costume are upset at the prospect of an actual Polynesian wearing it). That's what I mean when I say people have crossed the rubicon - "equality for all, regardless of race" and "stay away from other cultures with a race other than yours" seem at complete odds with each other. It's still siloing people off based on where they were born or what their skin looks like.
As for me, I'm off to wikipedia now to read about some Polynesian mythology