Mimikyu is really nice because its Disguise acts like a psuedo-Focus Sash and its typing isn't too shabby. Even outside of Disguise for an offensive mon it has okay bulk, too.
It's a nice stop to Pheromosa and can threaten non-Bullet Punch Mega Metagross if they're injured with Shadow Sneak. Granted, non-BP Mega Metagross is uncommon, but it's still relevant enough.
I actually think Mimikyu is in a decent spot in OU. Not astounding, but has a really nice niche that offensive teams appreciate and hyper offense teams can hate dealing with.