What effect will replacing 195 degree thermostat with a 180 degree thermostat on a dodge ram?

It could, the pcm is programmed for an engine that run at 195. With a 180 stat, the pcm will have to adjust fuel amounts because fuel burns faster the warmer it is. The pcm could set a code for either the cold stat, or a fuel sytem trim code. It isn't going to make it run any better.