We have lots and lots of options outside of the mutant gene. The beauty of the fictional universe is that the imagination need not be limited in any way.
For all we know, Monica's parents are Aspirants forced to live human lives by opposing Celestials. The accident just merely awakened her powers.
Or maybe the multiverse when it was first created also birthed a sentient abstraction that would later become Monica.
Or maybe Monica is a cursed god forced to live many life cycles in mortal form, completely ignorant of her true power and history.
Or maybe the Beyonder decided that it wasn't enough to merely look like a Michael Jackson clone, but he wanted to send a fraction of his psyche back in time to live as a Black woman in America. Monica is the product of that choice.
We could go on and on.