This is an interesting concept that requires a lot of thinking to figure out.
I would agree with you in our obviously overwhelming preference for fixed metal bracket appliances may be because there's little real-world cosmetic benefit compared with the other options. I believe that we as a group think there’s little cosmetic difference, but if I were a normy, I would assume that ceramics are pretty much invisible, exactly because of the point Charlie1086 made, they don’t look for them but we do.
I am thinking that people choosing colorful and bulky glasses or colorful ceramic braces has something to do with the fact that they only realize after they get them that they are very much visible, not pretty much invisible as they once believed.
The social stigma that has come with having braces has came down a long way than the 70s, at least from stories I’ve heard (I wasn’t around back then) which has definitely helped with more metal braces and freakishly large glasses.
Alternatively, I would argue with the fact that we’ve become “blind” to other things. At least for me, I don’t actively look for glasses, or hearing aids, but I see them anyway even if they aren’t Iris Apfel glasses. I would agree though that we see things that we are used to seeing, such as the baseball or police officer examples you gave. Pretty much what I’m saying is I don’t believe we go entirely blind to stuff, but we aren’t as perceptive to it as we are to braces.
These are just my thoughts on the matter, but I have no scientific evidence that these are correct. Feel free to contradict me.