Stock thermostat is a 195degF, I assume you are using that temp.
They also make a 205degF for the 2.3l Lima.
Easiest way to tell if thermostat is working is to feel the upper rad hose at radiator end.
After starting engine cold, feel the rad hose, it of course will be cold.
After engine has run for 3 or 4 minutes feel it again, should STILL be cold.
If hose is heating up with the engine then thermostat is not working like it should.
The radiator in all vehicles is there to get rid of EXTRA heat, some engines just don't generate alot of extra heat, 2.3l is one of those engines.
In the winter the radiator may not get very warm at all, because thermostat is barely open.
A thermostat it will start to open at +/- 3deg of rating, and be fully open at 10deg above rating.
It is very normal for the thermostat to open and then close again as cooler coolant flows in from cold rad.
2.3l coolant temp sender(for dash gauge) is located at the back of the engine, so gauge will read just above a 1/4 when engine is warmed up to 195deg.
Most engines have the sender near thermostat so gauge will read just below 1/2 when warmed up.
When engine has been running for 10-15minutes it should be warmed up, heater hoses should be 195degF, upper rad hose can be cooler since thermostat is not open all the way.
If one heater hose is hot and the other much cooler then heater core is blocked, plugged up.
1994 model Ranger heater cores are easy to change, and not expensive.
Cardboard in front of the rad is common in colder climates, but it is mostly to stop the air flow into engine compartment not thru the rad since thermostat controls the flow thru rad.
You will often see semi-trucks with zippered covers to block air flow into engine bay when weather is cold