Came back to bump this topic since new info has been found. While I know some people were in doubt and thought it was an awkward Japanese pronunciation, it isn't. It's a legitimate glitch. It was originally found by Gema_Yue, but now GameXplain are also vouching for it, saying that it works:
For those who want to try it out if they have a JP 3DS, you have to play as Villager on Yoshi's Island, and make all 3 CPU players Dr. Mario. Once Villager ends up above 100%, K.O. the Dr. Marios and if you manage to trigger the crowd chant, they will be shouting "Nana! Popo!" instead of Villager (or Murabito).
While I have not heard the Murabito chant myself, I don't think GameXplain (or any of the other people posting it) would post it if it was a simple awkward pronunciation, because they have more than likely checked the actual "Murabito" chant.