It's a thing in Germany as well. I remember that one of the reasons customers gave why they didn't want to go to Walmart when it opened a few shops in Germany was because they were freaked out by the happy act that employees had to put on.

The sort of always smiling, always laughing stereotypical American waiter or something would be perceived as juvenile, fake or not very bright over here.

I think you could apply the same in Poland. Maybe not "mental illness" but being a bit "off", maybe drunk or drugged. We also associate smiling too much with snake oil salespeople.

This is one of the reasons I get along so well with slavic and eastern european people.

The dynamic range is higher, versus having to figure out the difference between smiling and mega-ultra-smiling (as in the USA).

Most Russians I've know on a one on one basis were pretty jovial and fun to hang out with. They did thing people were "fake" and "smiled too much" in America though. I told them the American take on reality was as legitimate as their more cynical attitude. It's all in learning about culture and why people are the way they are. Russians tended to be very straight forward and say what's on their mind, and Americans will hold it back. There are pluses for both positions.

While that's definitely a Russian "thing" it's by no means confined to Russia.