There you go; Proof that the coolant is not warming up enough.
Don't know exactly for your car, but most modern cars run somewhere between 180 degrees and 205 degree thermostats.
When your thermostat is stuck open, the coolant doesn't stay in the block long enough to be warmed up properly, and therefore you've got no or very little heat from your heater.
Not only that, but your computer is reading those "cold" engine temperatures,
and may be ordering up a slightly richer fuel mixture.
What kind of fuel economy are you getting?
All in all, it isn't good for you or your motor.
Get that coolant changed and replace that failed thermostat with a new one pronto.