I was originally going an entirely different direction with this sketch, but you know how that song and dance goes. :V

Mochi will sometimes do this as an extreme expression of displeasure. Unfortunately, since all she eats are Twinkies, her farts just smell like cake and banana cream, so nobody really finds them offensive.

"I thought Mini-Michiis digested the entire Twinkie. Why does Mochi have a farthole?"

Remember that Mochi was a prototype and the Twinkie digestive process wasn't perfect. Waste still accumulated, and she has to get rid of it somehow. The consumer Mini-Michiis do not have a full digestive system due to the process being completed.