From the Super Mario Wiki
Mirrors appear in Mario Tennis Aces as a sapient species.
They first appear in Mirage Mansion where they stop Mario from advancing to the Power Stone in the mansion. They attempt to hinder Mario in a tennis match against Boo, but they fail and vow revenge afterwards.
The mirrors reappear in the Reflection Room to attempt to perplex Mario with a giant mirror. When Mario solves the puzzle, the mirrors are dismissed by their queen, and are not seen again for the rest of the game.