Strava isn't as popular in Italy as it is here in Holland or in the US, but it is growing. That being said, you're going to see a lot of foreigners in the segment results in Italy. That's mostly from people recording their rides while on vacation. You'll also see this in France as well. Nearly everyone I know here locally has at least one KOM in France. I have 7 in Italy myself. If you're riding in some far-off places well outside the major cities and vacation spots, you might only be seeing results from local pros and really hardcore riders, not sure. I rode up a small climb in Italy this past summer and when I looked at the leaderboard I noticed that many of the best times were set on the same day. Turns out it was the day that the Giro del Trentino passed through. There were only a few local guys.

As for the comparison of riding with local groups, I'm not sure exactly what you're asking but there are groups of every size and skill level here, just like there are in the US. I would say though that people who take cycling vacations to the South are probably a bit more serious, but then again that varies as well.