You can sometimes drive a car with a bad thermostat.
What I mean by that is that if you have a bad thermostat that is stuck in the open position then the coolant will flow through properly allowing the engine to stay cool enough so it doesn't overheat.
Now if your vehicles engine thermostat is stuck closed then it can cause your engine to overheat because the stuck closed thermostat will keep the coolant from flowing through the radiator and engine properly.
Replacing a vehicles thermostat can be done pretty cheaply on your own if you can drain the coolant down and then use some tools to get to the thermostat and then replace thermostat along with the thermostat housing gasket.
If your car is not overheating though and you think your vehicles thermostat is stuck open then you can drive with the bad thermostat.