I asked on Smogon if Z-Splash Mimikyu is viable on Smogon, and it's not viable. The user who answered the question, whose name is Katy, said:
I would not run Z Splash Mimikyu since it can run Ghostium Z or Life Orb better; the former makes you capable of hitting Steel-types for neutral damage such as Heatran and Celesteela, which are really relevant in the OU metagame and the latter can be used as well to dish out hefty damage with all the STABs Mimikyu offers. Z Splash makes you too reliant on the boost and furthermore you can plan on adding a Z move on another Pokemon without wasting it on Z Splash on Mimikyu and instead run Life Orb on it. I personally wouldn't recommend Z Splash Mimikyu as the aforementioned Ghostium Z / Life Orb are way better items on it without wasting anything important.
Source
Also, Mimikyu isn't viable in Gen 7 OU in general, as it's not on the Viablity Rankings. There are better physical wallbreakers in Gen 7 OU, like Kartana and Mega Mawile. However, if you still insist on using Mimikyu in Gen 7 OU, don't use Z-Splash on it because Z-Splash Mimikyu is too reliant on the boost, so you should a Swords Dance set with Ghostium-Z or Life Orb since Z-Splash is a waste of a Z-move slot. Basically, if you want to use a Z-move on Mimikyu, run Ghostium Z. If not, give the Z-move to another Pokemon and have Mimikyu run Life Orb as it boosts all of its attacks' damage output. Hope this helps.
