I mean to be fair, what Alex said was really fucking dumb for multiple reasons.
1. Mutant has never been a slur in X-Men canon, it's the excepted word for their people that hundreds of mutant characters have used and showed literally 0 offense when non-mutants use it. So Alex saying that people shouldn't use the word mutant makes him seem extremely out of touch with the mutant community, which raises the question of why the fuck he was chosen to be on an Avengers lineup specifically made as an effort on Cap's part to reach out to the mutant community.
2. If we apply real world minority statuses to his messaging it's dubious at best and dangerously flawed at worst. Like sure, in a Star Trek style united humanity we can put all the labels behind us, but Alex is a member of a race that is regularly subjected to both institutionalized, religious and casual oppression. Saying that we can't acknowledge you're a mutant when there are everything from lynch mobs to terminators hunting you seems a poor way to draw awareness to your plight. There's also the matter of cultural and racial pride, which is honestly fine if it's just about your love for your own history rather than your hate and fear of others. Who you are and where you come from effects certain aspects of who you are and you don't have to go full /pol/ or SJW to acknowledge that, just look at music, writing and all the other arts. That shit forms your perspective, not the sum total of it, but it's a part.
3. Motherfucker you shoot laser beams from your hands. You can level a city block in minutes. There are important conversations that need to be had about that and if you can't even engage with what you are then you're clearly not the person to be having these conversations.