My 1984 300D was like that, usually running ~60 C on dash gage. I removed the T-stat and tested in a pot of hot water. It seemed to work until I made careful comparisons against a mercury thermometer, IR gun, and other T-stats. It opened a bit earlier and responded more sluggishly. Replaced it and the car has run 80-85 C ever since. I recall mileage went up. So, yes they can kind of work, but need replacing. My other opinion is that you should always tell us the year when asking a car question.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans