Yes it could be the thermostat. Easy to check if it works. Remove it from the car and bring it in your kitchen. Suspend it from a piece of wire in a water filled pot and bring to a boil. It should fully open (80-85° before the water boils 100°C. Does your temperature guage runs above half? Other possibility is if your tank is overfilled. It should not be above the max mark when the engine is cold to allow fluid expansion.