Can engine overcooling and extremely bad mileage both be caused by the thermostat?

Ok so I now the thermostat being stuck open can cause the engine to run cold and effect mileage slightly but my engine never gets off C unless im towing a large load like a trailer or one time when i busted a radiator hose. Also I get 13MPG City and Highway when i should be getting 21ish The engine has 153,000 miles but runs great. it is a 2.3L 4Cyl Ford Ranger 1996 Automatic with Overdrive. like i said old engine but it runs well (all the plugs fire no pings or noises. maintenance is kept up except for the thermostat because on this particular vehicle it is burried underneath the fan and intake and is simply impossible to get to the bolts without pulling the fan and belt and possibly messing up the timing chain which is located directly above it on the right... this thing is a piece of work haha.