People seem to require something to fear - even in this world, where there is plenty of real evil to concern themselves with, they create a fictional counterpart. Or perhaps it's because there is evil out there - while most of them don't realize it, they may on a subconscious level, and use the fictional 'evils' to assure themselves that these things are simply stories.
Thus, since they assume 'witches' are 'evil', they make the fiction as different from their perceived reality as they can - the skin-tone, the 'ugly' appearance... If their 'monsters' looked like everyone else, there would be a small part of them that always wondered if the person beside them on the street was one, if they were in danger, and they don't want to wonder. If something is obviously frightening and unnatural, it can be more easily dismissed as fiction, and they are better able to reassure themselves that they are safe.
The reason for your particular story being targeted, I'm not certain, however.