We can refer to mutants but aren't humanity also technically nothing but a mutation according darwinism or evolution? We usually make a distinction between mutants and mankind but according to the theory of evolution the mammals are also a mutation(?) and therefore it's also technically correct (but maybe not ethical) to also refer to humanity as a mutation and refer to a "real man" also as a mutant since he has mutated from more primitive lifeforms. Why not?
All living things are the result of uncountable gradual mutations over billions of years, humans being no exception. But mutants aren't called mutants because of the fact that they have been mutated at some point or another, they are called mutants because one specific mutation has given rise to the X-Gene, which is the source of their powers. This mutation has surfaced many times over thousands of years and is not arbitrary.
The reason mutations and evolution is considered such a big deal in relation to the X-Men and Mutants overall is because they are considered to be a subspecies to humanity, Homo Sapiens Superior. Depending on the storyline or writer this could be portrayed as mutants being the next obvious step in human evolution, or that it is a competing species that will supplant humans, or something similar that puts them at odds with the rest of humanity.