Do you need a thermostat on a 95 Honda Civic dx?

Yes, you most certainly do. If you have no thermostat installed then the engine will take a very long time to reach full operating temperature and may never get to the proper temperature. The thermostat allows the engine to warm up quickly which helps prevent engine wear, reduce engine deposits, and emissions. The thermostat keeps the engine at the proper operating temperature for the best performance and best mileage. Running a modern computer controlled engine without a thermostat is a very bad idea.