It is definitely ableist/sanist to imply that erroneous people are mentally ill. But be aware that 'd*mb' is an ableist slur referencing Deaf/Mute people and equating people who can't speak, with 'stupidity' (which is itself an ableist pejorative, when really they mean 'willfully ignorant'). https://thebodyisnotanapology.com/magazine/stupid-is-an-ableist-slur-breaking-down-defenses-around-ableist-language-liberating-our-words/