BASIC Troubleshooting

My 1997 LHS doesn't seem to be putting out the heat that it should on
these cold winter days (just below freezing).  I notice that the temp
gage reads much closer to the "C" end of the range than to mid-range
(see sketch below).  I checked the coolant level in the reservoir and
it seems to be fine.

Might this be a thermostat problem?  If the thermostat was stuck open
would it cause these symptoms?

TEMP GAUGE:

C . . . | . . . H

(This is after driving the car for >30 minutes.)