Mortimer is one of the secret characters that can be unlocked in the Mickey & Friends section of Disney Crossy Road! In order to unlock him, you have to be playing as Mickey. As you play, you may cross upon a field. There is a chance that Mortimer will be standing in the middle of this field. If you see him, all you have to do is walk up to him until he pulls out his guitar. Once he does that, you will have unlocked him!
If you know how to unlock any other Disney Crossy Road Secret Characters, make sure to tell us in the comments below! Thanks to DaMobileMob for this info