Mime evolves into Mr. Rime when it reaches level 42.

Does Mr. Mime evolve Pokemon?

And the good news is that you'll be able to evolve Galarian Mr. Mime into Mr. Rime in Pokémon Go by just completing the Special Research as this eventually provides 60 specific Candy as a reward.

How do you evolve Mister Mime?

Galarian Mr. Mime is a Psychic/Ice type Pokemon and evolving it is rather simple, as all you have to do is keep raising its level until it reaches 42. Once it reaches 42, it will evolve into Mr. Rime.