This is what I always assumed... the mask doesn't actually have a mouth.
It's also been assumed the Indigo Islanders were inspired by Polynesian cultures... BUT exactly which real-world culture(s) have masks anything like Kahuna's?
Maybe if we establish the direct cultural inspiration, we can further determine whether the mask is intended to have a mouth or not.
Here is a side-by-side comparison by Noriart of the recent Tribal Warrior and Kahuna... the newer minifig seems a lot more grounded in reality whereas Kahuna is more fantasy.