Mutations are the potential next step in any evolution; in that a living being who is capable of procreating more often due to a mutation will out-live its parent species. Like being accidentally born with a different kind of claw that allows it to hunt more efficiently, or being born with an opposable thumb that allows more efficient climbing into trees and such shit. Or having a larger brain that allows them to figure out how to use tools.
A current real world example I can think of is Ozzy Osbourne, who has a random mutation that allows his body to break down alcohol more efficiently, which is why he can drink so much. If say every water source in the world would suddenly become vodka, he'd be one of the few people on the planet who could father children who don't die of alcohol poisoning, making him the next step in evolution. Otherwise he's just another freak.
Mutants in Marvel are kind of like that, they all have a random set of mutation and whichever one is more efficient will have more kids and rule the Earth. Obviously it won't be the rock golem with no penis; more like the runt who heals so fast he can take a nuke to the face and doesn't even have grey hair despite being 130 years old.
The bullshit part in Marvel is that mutants have the x-gene that enables them to have random superpowers. They wouldn't need that gene, they'd just be normal humans who are born with an extra appendix or claws or such, and they'd be still humans. You'd need one such identical mutation grow in a large enough numbers to replace the parent race for it to be the next step in evolution. But just being a mutant does not make you the next step in evolution, any more than any child born is the next step in evolution.