Theory: Delfino Plaza's Flood cleared up the Graffiti

Abstract
The flooding of Delfino Plaza cleans up the last of Bowser Jr’s graffiti.

Arguments

 * Argument 1: One Pianta in Delfino near Mario’s wanted poster complains about how someone ruined his wall, indicating the graffiti that Bowser Jr. painted everywhere. He will continue to complain about his wall right up until the final Shadow Mario Episode, the last point that Mario can roam around Delfino Plaza before the flooding. After Bowser is defeated however, the Pianta claims that the graffiti is gone, proving that the removal of the graffiti had to have happened between the flooding and the post-game.


 * Argument 2: Once the post-game is reached, one of the police officer Piantas will be informed by Mario that the flood cleaned up the whole town. While the rest of the townsfolk will talk about how the island is clean again, this is the only explanation given in Delfino Plaza for how, and it would make sense for Mario to explain to the police officers who made sure he was doing his job why the island is clean so that they can release him.