How much of mimikyu is costume? I mean does it’s body extend to the head or is it just in the body of its costume and the head is fluff?

The head is fluff, yes, and the mimikyu is located entirely in the body. Researchers know this because mimikyu allow us to squeeze and touch the head and even scan it with x-ray and ultrasound scanners. We just can’t actually take the costume off entirely and look at mimikyu’s true face for ourselves.