Considering so many people are mentally crippled in some way or another, I don't give it a second thought when I run into someone with DS. I've taken care of mentally handicapped people in the past- including a severely autistic girl & another girl with DS. It's not entirely alien to me. I will admit, I occasionally will have an initial, subtle disgust response, at times, due to encounters with handicapped people who didn't have a fair grasp on personal hygiene or the meaning of personal space. Yes, it's sad some of these people are not looked after properly/can be germy like babies, to some degree-- but I'll clarify that my visceral disgust response to potential exchange of pathogens is natural, & not a judgement on the person, by any means. And sure, there are plenty of unsanitary 'normal' people, but my encounters with this concentrated group have been most consistent, thus the involuntary biological reactions on my part. My feelings are not out on display, as most of my reactions tend to be muted by default, anyway, so generally people are none the wiser.
Overall, I am indifferent. I don't bother with pity; that doesn't accomplish anything-- there is nothing to accomplish, as far as I'm concerned. The reality of the situation is that they exist. So do I. Like any other being, I wish them well. And that's pretty much that.