Anti-vaxxer carries a social dismissal and a stereotype that isn’t true. Mental midgits is childish and as such is dismissed. Both are name calling however one is used to stereotype and discredit while the other is equivalent to dooty head.
And I’m not going to try to shame shutup you by condescendingly saying, “I thought you were better than that”. You clearly only care about name calling that helps/hurts the argument or group with whom you agree. Great way to “other” those who don’t agree with you.