Mime Jr. doesn't really have much of a viable moveset. I can see him being a Pokeball Pokemon easily though.
Mime Jr. has a great and distorting movepool available for him... He has one of the BEST movepools for a baby pokemon! In fact... Here's a moveset for him:
B- Mimic- Copies last used neutral b move and keeps it until he loses a life.
Over B- Barrier- Mime Jr. creates a barrier in front of himself that can take up to 10% damage. It goes away when it takes 10% damage or if it stays on the field for more than 15 seconds. No more than one can be on the field at once
Up B- Baton Pass- Mime Jr. throws a baton upwards and if it hits someone they swap places. Has great distance and almost no lag but if it doesn't hit anyone Mime Jr. won't come back.
Down B- Teeter Dance- Mime Jr. starts dancing oddly and music notes fly off of him. If a music note hits you your controls will be reversed and if you try to use a move you will receive damage like pichu. This lasts for 5 seconds. Also this gets rid of flowers on you like by the lipstick and gooey bombs.
Final Smash- Trick Room
Everyone swaps position and they get their speed halved and weight doubled. This move lasts 10 seconds. Mime Jr. can move normally and rack up damage. While everyone is slow Mime Jr's attacks do twice as much damage as they normally would, though still retain the same knockback.