Every new generation of Pokémon has some kind of Pikachu-adjacent Pokémon. Generation VII, the Sun & Moon games, actually had two. There was Togedemaru, a hedgehog Pika-clone, and Mimikyu. Mimikyu is really some kind of terrifying eldritch horror that wants to be loved, and dresses like Pikachu to get that love. The tactic worked, and now Mimikyu is one of the most popular Pokémon of all time. It's only natural to see it make a comeback in Sword & Shield.
Where To Find Mimikyu In Sword & Shield
Mimikyu is exclusive to the Wild Area, and even then only appears when there's fog. Go to Bridge Field when it's foggy to maybe see Mimikyu wandering about. Or look in Giant's Mirror in the tall grass during fog.
What Mimikyu Does In Battle
Despite not evolving or Gigantamaxing, Mimikyu is a good fighter. That's largely thanks to its Ability, Disguise. This makes the first attack done against Mimikyu in battle deal zero damage. Essentially, it gives Mimikyu's Trainer a free turn to do whatever they want.
That free turn can be used in a variety of ways. You could use Will-o-Wisp or Thunder Wave to cripple the opponent. Or you could lay down Light Screen to protect your team. Another option is to use Swords Dance to start the fight with a Mimikyu buff. As for what attacks to give Mimikyu, go with physical moves like Play Rough or Shadow Claw.