From the Super Mario Wiki
Super Mario Galaxy
Topminis first appeared in Super Mario Galaxy where they were enemies in several levels where Topmen appeared (mostly in the Battlerock Galaxy and the Dreadnought Galaxy). They were typically found in groups of two to three Topminis and they tried to spin into Mario which would simply push him backwards. This move, though incapable of damaging Mario directly, can push him into some sort of hazard which will damage him.
Topminis can only be defeated by a spin which will cause them to explode into one Star Bit that Mario can collect. However, they could also be destroyed by shooting a Star Bit at them, which would give three Star Bits instead of just one. In many areas after the Topminis are defeated, a hole will appear and more of the enemies will emerge from it.
Super Mario Galaxy 2
They appear in Super Mario Galaxy 2 in the Space Storm Galaxy. They behaved just as in the previous game.
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