Delta and Epsilon-level mutants. The top tiers described above are the most visible ones that affect public opinion, but these are the vast majority of mutants, making up 2/3 or more of the people who go through mutation. And they are sitting ducks for abuse. Deltas have some interesting powers but aren't nearly as strong as the tiers above, so an angry mob coming at them is going to win. And Epsilons... they get the worst draw of the lot. Poor—if any—powers, and major physical hardships. A good comparison is between Angel, a drop-dead gorgeous X-Man with huge white wings, and Beak, who also got the whole "bird mutation bit." It didn't work out quite as well for him as it did for Angel. Other mutants have died when their powers emerge; there was this totally heartbreaking story about Northstar trying to get a kid back to Xavier's before the kid's powers kept building and blew his own chest apart. He didn't make it.

So, between the sense that mutants are trying to displace humanity with their showy awesomeness that no one else is allowed to have, coupled with the fact that if your kid does mutate then the odds are that they would actually just be vulnerable and perhaps horribly, horribly deformed, then there's a real drive to stomp out everything to do with mutation so "normal people" never have to deal with it.

And that is a short-but-not-really version of the biggest form of bigotry in the Marvelverse, and the mindset around which these kids have been raised!