Before servicing, arrow of the gauge was always in the middle while riding.
I did a valve adjustment (intake was overtightened - now it's 0.10mm, exhaust was 0.20mm instead of 0.15mm).
Also, I have replaced a leaking thermostat and its cover.
Now, arrow always looks like this, is it a good temperature?
I did 20km in "full throttle" mode, after that I was able to see it pointing to the middle after few minutes of idling.
Thank You very much for help and participation!