Is it safe to play ball with mallet fingers?

Yes with splint on: Mallet finger occurs when the outermost joint of the finger is injured. With mallet finger, the tendon on the back of the finger is separated from the muscles it connects. So if you have the splint to protect it i think is fine.

Generally no: If you are actively being splinted, playing ball may affect the results of your outcome. It depends on if you have had surgery versus how many weeks you are into your treatment versus what type of splinting you are doing versus what sport you are referring to.