It creates a copy of itself that does nothing. It'll go away when hit with enough force that equal a quarter of your actual Pokemon's HP. Although, that means recoil would do no damage to Mimikyu XD meaning the splinters and shock waves would go into the doll and not Mimikyu. She doesn't have a lot of HP, so if she did hit your Snorlax, she is just one Crunch away from being knocked out. You just have to get through her Disguise first.