even in gen 3- mimic will become mimic after a battle. a pokemon can not have a move it can not learn (unless hacked) out-side of it's tm and level moves to be changed/altered out-side of a battle. in-battle, mimic is an effect, not a permanent resolution to an outcome of a battle that has a lasting effect. same as like transform- your pokemon will revert back to it's original form. Sketch, on the other hand, is the only attack that will remain under specific circumstances and breaks the after-battle conclusion for a change to take place to a pokemon's move-set. since sketch is specific to smeargle- it I the only programmed pokemon to have the privilege of the one-time after battle effect of having a move replace sketch.