As noted PU doesn't actually copy the target (as it's description may imply), but instead acts as though you played the target from your hand. Hence you get a random mutant when you 'copy' a mutant. This interaction is even more clear when you 'copy' a Chimera and it merely combines all your creatures into a new chimera.
If you copy your chimera, if you have any other creatures, they are absorbed into a new one. If you copy the opponents chimera, all it does is combine your own creatures into a chimera.