Meh. I kinda hate the idea that mutants have been in the background during the MCU going completely unnoticed. It sounds as dumb as The Eternals sitting around like lazy bastards, doing nothing while Thanos tries to destroy Earth. And how on earth do you do Wolverine, if he’s a teenager around IW?
I said majority of mutants are teenagers. Some have been around for longer, Wolverine and Apocalypse have been around for a very long time. Even the first X-Men movie by Fox starts with a meeting that talks about mutants in hiding and that they are real and that they exist. Basically suggesting that mutant sittings are becoming more frequent.
You could place the first X-Men film into the MCU and it would function fine. Having the majority of mutants as teenagers is a good excuse for them not being around during Infinity War and Endgame, too young and inexperienced to help.
Realistically though, in Infinity War there is only the initial attack in New York which is a short time frame, not enough time for mutants to arrive. Then there is Wakanda at the end, mutants wouldn't be there because they done know about Vision or the stones.
Then in Endgame, it's literally just the last battle at the end at Avengers HQ, again a short time frame and not enough time for any mutants to arrive.
As for Eternals, no one knows why they haven't interfered and it's silly to judge that film based on a teaser. My understanding of them is that they are gods and think themselves above humans and don't care for them. Not only that, they also have their memories wiped and so don't know that they are "eternals".
I don't see what introducing mutants by the multiverse brings to the table.
Do they stay in their own alternate reality? If so, then they aren't really being introduced into the MCU it's a separate reality. It's kind of pointless not having them interact with other MCU characters.
Or are you saying they are brought to the MCU reality from their alternate reality? If so, how many mutants are meant to travel to the main MCU reality? There are hundreds of mutant characters in the comics. Are they meant to use the multiverse gimmick every time they want to introduce a new character?