From the Super Mario Wiki
The Pianta judge is a Pianta who holds trial for Mario when the Isle Delfino locale is vandalized by a figure resembling him during the events of Super Mario Sunshine. Although Peach tries to object, the Pianta judge overrules her objection and pleads Mario guilty. She then orders Mario to clean up all the graffiti that Shadow Mario created during his vandalism spree.
She is not seen later in the game.