Surprisingly good, actually. Never really doubted poor, poor Mimikyu, and it looks like it could be a good RU, UU, or potential OU. For one thing, it's Disguise ability extends beyond some cheap gimmick. It has some tactical value, seeing as it gets access to Swords Dance and Hone Claws to boost its decent 90 attack, and it can take a meatshot for its teammates to keep the team running in good condition. Another upside is the fact that Disguise doesn't break from indirect damage, meaning that you can come in on Stealth Rocks safely and still take a meatshot. The only downside is that it only activates once, so you have to use it wisely, or else Mimikyu might become dead weight afterwards, depending on the situation.
Everything else is still quite decent. 55/80/105 bulk is better than I expected, but it won't allow you to take multiple strong hits (neutral or otherwise) after the Disguise breaks, so best be aware of that. 96 speed is just a few points shy of the crowded 100 base speed tier, but is far from being horrible, and allows it to outspeed non-choice scarf boosted Krookodile, among others.
Movepool is decent, though Mimikyu can't make use of the nice Special movepool it gets due to its mediocre 50 Sp. Atk, so run Choice Specs if you're feeling curious about a using a Special Mimikyu, but then again, take this with a grain of salt. I assure you...
Other than that, getting Shadow Sneak is a plus, along with Leech Life, which provides some decent recovery. Pain Split's another option if you want to take advantage of Mimikyu's low HP to heal to great amounts, but it can have its downfalls. Swords Dance, like I said, is a huge plus for Mimikyu, as it can boost its decent base 90 Attack to hit harder, though Bulk Up can be used to patch up it's average 80 defense, but earn a lower attack boost. Wood Hammer's an interesting choice if you want to hit decently hard and punish switch-ins of the likes of Azumarill or Tyranitar, who can be OHKOed and/or 2HKOed by this move. Shadow Claw is another STAB move that lands crits more often, so feel free to use it alongside Shadow Sneak. Mimikyu also has a decent support movepool, with Will-o-Wisp, Thunder Wave, Infestation, Taunt, etc, and it's decently fast enough to use them, though do op for a bulkier spread if you wish to use them effectively.
All in all, Mimikyu is no joke, and it surely has what it takes to take on top threats with the right tools. Having decent STABS, as well as being neutrally affected by Poison and Dark type moves due to way of that Ghost/Fairy typing. Steel and Ghost type moves are the only big worries, but it's still neutrally affected by Dark type moves, as I said, so be wary of mons with Knock Off, especially since you could lose your item and take a good chunk of damage. Having access to an interesting support movepool with the speed to back it up also aids its teammates and allows them or itself to set up in Doubles as well as Singles, so make great use of that strength. When played right, Mimikyu could be a game changer, but played wrong, and it becomes dead weight...
Sheesh, this is kinda messy looking. I'll edit it when I get the chance.