If you look at the new Ultra Sun and Ultra Moon Trailer, it looks really empty, like a floor or some sort. No inside
No one knows for sure, but in one episode of the Sun and Moon Pokemon anime (The Sun, the Scare, the Secret Lair), Mimikyu takes off its cloak. It looks like Mimikyu's inside is sort of a black blob.
Mimikyu has an amorphous shadow filled with nightmares and dreams of Pokemon that have looked inside Mimikyu.