If you have mono can you get your lip pierced why or why not?

No. Your body is fighting an infection and a serious one at that. Piercing while you are sick let alone fighting Mono is at best a stupid move. You tax the bodies ability to heal by loading it down with another thing to heal. Professional body piercers will not knowingly pierce anyone who is sick or fighting any form of injury, to do so is irresponsible and negligent.