This subject has come up before and I think many, if not most people, feel the same way. More strongly towards Lola, but Claire as well, especially as she got older and became more freakish looking. It's not just the fact that they are strange or abnormal looking. That's part of it, but there are other disfigured people out there and while some may evoke initial reactions of shock or even horror if they're seriously deformed, few induce the feelings of sickness in people the way the Hartley girls do/did. I think one reason is that their brains are as damaged as their bodies, so they truly lack almost all of the abilities that make us human. I think if they were even capable of smiling,people would feel less repulsed by them. Im talking actual social smiling, not that twisted grin-like expression that forms on their face during a seizure. But all Claire could do was give that bug eyed stare and gape open her mouth like a dying fish, while Lola just bleated and rooted and she cant seem to even do those things anymore.
Finally, and I believe Ive mentioned this before as well, but I think their mother has a lot to do with how people view them. Over the years she seems to have purposely gone out of her way to make them appear as gross as possible by writing in the most sickening detail about overflowing diapers and all their other nauseating bodily emissions. All that talk about vomit and shit and feta cheese-stinking hands made people feel much more disgusted than they would have otherwise.