Hatterene's base 136 Special Attack is much higher than Mimikyu's 90 Attack, making it a more potent attacker right off the bat. However, Hatterene is much slower than Mimikyu, meaning it'll need to take more hits before it can get anything done. Both of them have varied movepools if you're willing to grind TRs, but both level-up movepools work fine as well.
Overall I would generally choose Mimikyu, as though it's offensively weaker, 90 Attack is still fine for in-game. In addition, Ghost is generally more valuable than Psychic, and its Speed coupled with Disguise means it won't have too much trouble taking out opponents. Here's a set:
Mimikyu
Ability: Disguise
- Shadow Claw
- Play Rough
- Drain Punch / Slash
- Leech Life / Wood Hammer / Shadow Sneak
