From the Super Mario Wiki
The Graffiti Ball is Bowser Jr.'s Star Pitch, from Mario Super Sluggers. This Star Pitch involves Bowser Jr. covering the baseball with paint, and then throwing the ball at the batter. During the process, the batter's vision gets heavily hindered by the paint, making it harder for them to hit the incoming baseball. The baseball does not move very fast though, so with the right timing, it is possible for the batter to hit the baseball.