As I flesh out ideas for my own race categories in GWW, I thought I'd capture some quick thoughts about how mutants are presented in other settings.
With so many sources from which to field for inspiration, it's sometimes hard to pin down exactly what qualifies as "mutant" in a post-apocalyptic setting. In Marvel's universe--as with the X-men and other X-groups--the term "mutant" applies to a new class of human being called "homo superior," or a human who's genetic code is in someway altered to acquire any sort of superhuman (and I'll throw in here "non-human") abilities.
In the Marvel universe, Iceman with his power to create ice constructs, Jean Grey and her ability to read minds/use telekinesis, and Wolverine with his incredible healing ability are considered, mutants.
Switch to something like Total Recal where the mutants are more of a traditionally "deformed" depiction. Kuato, Benny, and a host of other melted-face Martians fill that role.
In GWW, I'm using it in a much broader sense--to mean an individual or group, who's appearance and makeup deviates from an ancestral species of origin. This includes hybrid types (e.g., any sort of were-creature), to the Cyborna races, which are at least partially robotic. There are enough of them that have, in some way, deviated from their collective original forms that they qualify as a new species or subspecies. I guess Teenage Mutant Ninja Turtles: After the Bomb would be a good example of this style.
It doesn't really matter how they became a mutant--they didn't need to be the result of some sort of evolutionary process. A mutant could be an individual who came in contact with some foreign, mutanagenic substance that caused them to change. In GWW, Spider-man would easily be considered to be a mutant, even though his classification as one is oft debated.
It might be easier to state what mutants aren't. They aren't pure strain humans, called Valids-- or flora or fauna, for that matter. They aren't purely robots, created for some specific task. They aren't unmended (reanimated dead). They aren't extraterrestrials. At least--in all of the above cases--they aren't until they begin to deviate from those baseline origins.