Generic* Voodoo tribe, I believe. Shrunken heads and all that business.
*I say generic because I have no idea where these tribes originate, but as the World of Warcraft trolls have Jamaican accents, I'll take a guess at Jamaica.
Edit: Although it's worth mentioning there's a totem pole in the village, possibly connecting him to Native American culture just like Humba Wumba.Edit 2:
Okay, I did some digging for you. I believe Mumbo is inspired by traditional Zulu warriors on top of the voodoo (both originating in Africa, but spreading to Haiti etc. which explains the troll accent in World of Warcraft /off-topic).Zulu Warrior (+ Shield) + Mumbo:
Notice that Mumbo has the traditional Zulu shield hanging behind him on the walls and two African masks on the podiums beside him. He also has a similar loincloth and similar feathered head-wear. Case solved?
Additional Edit: The beta name for Conga the gorilla was Congo. Congo is a country in Africa; not to mention that gorillas themselves are only native to Africa. The also originate from Africa (alongside Australia and South America). Putting these clues together, I'd say Mumbo's Mountain is itself loosely based on Africa with Mumbo; however, Stonehenge is English so it's not got any one particular theme. The level barely resembles Africa outside of the links I've made (). I guess it's like Clanker the "whale" having gills like a fish, but a blow-hole like a whale. Things are inspired in the Banjo games, but not always true to life; the devs threw in anything they fancied. Game-play came before logic, I think.
I have too much time on my hands, don't I...