I really don’t want to sound wrong on that question. Please, don’t kill me!
I have noticed that many people with Down syndrome wear similar clothes. Guys usually wear a polo shirt tucked in waist-high pants and girls seem to usually wear a dress. Of course, they are highly functional individuals that can do pretty much anything the “normies” do. Few months ago we had a great actor here in Brazil who had DS. He is now married to a woman who also has DS. The thing is that when they are being cared by parents or guardians it looks like they almost wear an uniform. So… Why do parents and guardians do that?
You don’t really see that in my area, SW Texas, here they normally just wear what other people do (tshirts, jeans, shorts,etc).
It could be a sensory issue, could be parents/gaurdians want to make sure they look nice in the hopes others may treat them better or it could be just how they want to dress.
I don’t think it looks any more like a uniform than T shirts and jeans worn by teenage boys looks like a uniform. But it does often look like the uniform of an older generation than the people wearing it - which is very possibly the result of that older generation choosing and buying the clothes.