The greatest mystery in Pokemon: what is under mimiku's cloth?
Although there hasn't been an offical answer yet, there are a few things we can guess based on what we know. Mimikyu is likely a spirit of a Pokemon or human (like Yamask) and no larger than 0' 8'' which is the size of Mimikyu. It also has long dark appendages it can lenghten or shorten at will which it would have from being a ghost type. If it is the spirit of a Pokemon that Pokemon may have likely been at least part Fairy which would explain Mimikyu's typing. It may be shorter than 0' 8'' as the eyeholes in the costume are below the Pikachu face that is on the costume. Also implying this is how the head of costume snaps when attacked by another Pokemon. In the anime it doesn't like Pikachu which could mean the Pokemon felt Pikachu (as a species) did something that Mimikyu despised to it such as (but not limited to) making Mimikyu feel outshadowed because it was less popular, being stronger that Mimikyu (if it wasn't originally a Mimikyu) or something I didn't even mention.
In short, based on what we know about Mimikyu, we can presume it is a spirit of a Pokemon or human with ghostly appendages it can change the length of at will. If it's spirit of a Pokemon it was likely at least part fairy before becoming a Mimkyu and was 0' 8'' ft or shorter and didn't like Pikachu for reasons that might include Pikachu being more popular and/or stronger.
Hope I helped :)