Normal operating temp is about 210 on the dash gauge. When you see less, the thermostats even aren't open yet. If you start seeing over 225, then you might issues. I have lower temp thermostats that are intended for racing, but they won't fix overheating, and might lower your fuel economy. The engine will protect itself from actual overheating, which is about 250 deg.
Diesels actually like it hot. It gives you a more complete burn.