I started a new Ultra Sun run recently and traded over a few Pokemon I had in eggs to be on my team, one of which being Mimikyu. When I looked up how to get the Z crystal for Mimikyu, the site said to catch a Mimikyu inside the Megamart and interact with the other one to get the crystal.
Since I already have a Mimikyu on the save that I hatched from an egg, do I still need to catch a Mimikyu in that location to get the Z crystal?
It does not matter where your Mimikyu is from, you can still use it to access the Z-Crystal. You do not need to have one caught from the Megamart specifcally, because that's the only place one can be caught, and it doesn't make sense that GF would just not let you use one obtained from an egg or a trade.
My source is experience, I used a Mimikyu I caught months before I did the event for the Crystal.