They look noticeably different but aren't that rare. So I usually go "what's that? Oh, DS, okay, whatever" within a second or two.
BUT...
... despite all my parent's efforts to the contrary and although I could and probably would passionately argue to the contrary in any debate, I secretly measure my own (and to a certain extend also other people's ) subjective "worth" by intellect. And I have trouble imagining how anybody could live a life worth living without advanced reasoning skills and the ability to engage in complex thought. Careful, I am not saying this is a requirement for a worthy life, not at all, I am saying
I have a hard time imagining myself in the shoes of somebody who is so different in that regard. I have no problem at all with just about any kind of physical disability or deformation because to me the body is mainly an unimportant shell. Mental disability however somehow strikes a chord with me and can scare me a bit for that reason.
So while I do not pay much extra attention to people with mental disabilities (as individuals) when I see them they sometimes do trigger a whole chain of thought that can go on for several minutes (about the role of intellect in people's lifes, what makes us human, subjective and objective aspects of quality of life, my own subjective values, the pros and cons of those standards, what I would do if I had a child with a mental disability, what I would do and how guilty I would feel if it turned out I couldn't emotionally connect to them enough because of the lacking mental connection, etc.) So yes, they can trigger uncomfortable thoughts and feelings.
And, as others have said, whenever somebody looks different and (though very shortly) attracts my attention for that reason ...green hair, bunny costume, two noses, you name it...I try to neither stare nor look away