How to Get Mime Jr. in Pokemon Legends: Arceus 

You can find it in any weather at any time of the day.

On the Obsidian Fieldlands, Mime Jr has a low probability of appearing in two locations.

The first location is a large grassy field near the Horseshoe plains.

The second Secret Spot is located directly to the right of Sandgem.

Mime Jr. is quick to flee. Use tall grass to sneak up on it.