What can I do if I was permanently scared by an employee at work from a joke. Can I file a lawsuit and would it be worth it?

The simple answer is yes.  However, you should evaluate the seriousness of your scar and the damages it has caused in relation to your chances of actually recovering from the co-worker.  Additionally, you should keep in mind that you have only 1 year from the time of your injury in which to file a lawsuit, otherwise your claims may be time barred.  You may also consult an attorney the routinely handles workers compensation benefits as it may be considered a work injury.