To catch Mime Jr. in pearl, you can import a Mr. Mime from lesf green or fire red (obtained by trading an abra to the guy with the game boy in the building on Route 2). Once you have the Mr. Mime have it hold a lax incense and just breed it with a ditto.
Another way to catch it is through going to Mr. Backlot in the pokemon mansion. He will tell you of a daily pokemon. If it is Mime jr., just go into the Trophy garden to claim your prize. P.S. It's pretty rare.