Under a narrow interpretation of Safety, sure, a Thermostat failure may not be an issue: there is little chance of serious injuries.
Under a broader view, the T-Stat becomes an issue. It is a distraction, added stress, could cause a non initiated to worry and stop on the side of the road, which is not without danger, emission issues from engine operating out of spec etc, etc.
When filing an NHTSA report, you can get their attention by indicating via check boxes if your case resulted in an accident or fire, but there is no indication that an incident shouldn't be reported if it doesn't cause accident or fire.
There is a good overview by Edmunds HERE, of Categories of instances reported to the NHTSA, where it is shown, for example, that even relatively minor issues like gauge malfunctions can be reported.
The Power train category even includes fluid leaks. Which indicates that the infamous coolant leaks should be reported as well.
For the heck of it, I asked the NHTSA where they'd stand about us reporting benign instances like T-Stat's. I couldn't get across that it was merely an inquiry about reportability......they replied with the Hotline number to call for filing the complaint!
Chances are good that more credence would be given to a list of reported ST defects extracted from the NHTSA database than to one pulled from our beloved BB.
