Interesting. So the educational filter should make people in Slovakia appear smarter to Americans (if they notice this country at all) simply because the worst stupidity won't get translated, and the lowest-class people will not travel to USA. You will not be regularly exposed to things like this.

On the other hand, this effect is probably much smaller than noise created by random American journalists or bloggers writing made-up stuff about Slovakia, or depictions of "Slovakia" in movies (example here, or shortly here). If for whatever reason a popular writer would decide that Slovakia is e.g. inhabited by vampires, there is pretty much nothing we could do about it.

All the people I've every met from X are highly-skilled immigrants

Maybe the right question to ask yourself when you meet a smart immigrant is: "Why did they have to leave their country?" Probably not polite to ask them, but you should assume there was a reason. And if the answer seems to be "poverty", well, poverty is usually caused by something, so unless the country is just one huge empty desert, there are other things wrong there, too.