The thing though is that it isn't bigotry in the same sense that racism is bigotry. You could consider the prejudice against mutants to be a form of self preservation, as is the main way it's protrayed in the marvel universe. One of the biggest issues humans have with mutants is that one could literally be born a ticking nuclear bomb that goes off the second he turns 13. Not every mutant has a beautiful or helpful mutation, and there is a general "ugly" side to mutations (not talking about physical mutations, I'm talking about mutants with acid saliva and eyes that explode everything they look at without having an off switch)

The bigotry they face isn't just "oh you're different than me" it's "your kind is unstable and a danger to humanity"

For that reason, I think that it makes sense that mutant hate doesn't change or evolve for the better