Where is the thermostat in reference to the windshield reservoir?

Hi ktaran. It's unlikely that the repair to your thermostat directly led to problem you are having with your windshield fluid flow but moving around the engine compartment can sometimes crimp or disturb other systems. If you live in a cold weather area like I do your windshield wiper fluid is life itself. Go back to the mechanic and have him run down the issue with the fluid. Once he figures that out it will become pretty obvious whether the two are related or whether they were unhappy coincidence. He'll probably just take care of it either way if it's a small problem. Good luck!