Lack of random environments

The cat is pondering the mutations hypothesis at the moment and he has a related question to ya'll.
It is postulated that all mutations are random and if the moggy understands the proposal correctly that means all mutations are random in respect to fitness. Where fitness is the capacity to make as many of copies of oneself as possible.
Now the opponents of the scenario point out that most known mutants are creatures with seven heads growing out of paws and similar monstrosities.The next bit of the proposal though is that it is the environment that determines fitness randomly brought about by chance mutations and that the environment is anything but random eliminating all which does not serve to fit in it.The cat's question is: does that mean that the three-headed monster critters are just lacking a proper environment to adapt to and to be fit in? Could there are be an environment they are the fittest for? It must be so if there is no intrinsic damage or benefit brought by a mutation but the selecting environment alone decides who is fit and who is not.
Thank you in advance for your attention and clarifications needed for enlightening of the stupid feline on that delicate issue.Edited by Alfred Maddenstein, : No reason given.Edited by Alfred Maddenstein, : No reason given.