From the Super Mario Wiki
Mirrors are a sapient species of mirrors introduced in Mario Tennis Aces.
They first appear in Mirage Mansion to 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.