X-Men: 10 Theories For How They Can Join The Marvel Cinematic Universe
8. All Those "Snaps" Led To Some Big Changes In The MCU
At the end of Avengers: Infinity War, Thanos unleashed the power of the Infinity Stones when he wiped out half of all life in the universe. Then, he did the same again to destroy the Stones, before The Hulk used them to bring back the dead. Finally, Iron Man used them once more, this time to wipe out the Mad Titan and his army.
Each of those "snaps" led to a massive surge of energy, and it's hard to imagine those didn't lead to some changes in the MCU, especially when the individual Infinity Stones hold so much power (they brought The Vision to life and made the Scarlet Witch this shared world's strongest superhero).
As a result, the sudden emergence of mutants could be explained by any one - or all - of these uses of the Infinity Gauntlet. It would be a clever way to explain where they've come from, and there should definitely be longer lasting ramifications for Earth's Mightiest Heroes beyond the death of Tony Stark. The creation of mutants would certainly fit the bill in that respect.