It "can" help to a degree (no punn intended.) The thermostat is your "temperature regulator." It opens and allows coolant to flow through the radiator. So if you have a cooler T-stat, it'll open at a cooler temperature and start circulating coolant into the radiator sooner than a hotter one. If the radiator's able to extract that extra heat, then the engine and coolant will run at a lower temperature. Once the radiator can't get rid of the extra heat, you've exceeded the capacity of your cooling system and the thermostat isn't going to make any difference.
Therefore, the T-stat is just one of a few variables that determine what temperature your engine's going to run at. The two other major parts (if everything is working properly) are the radiator and the outside air conditions. Hope that helps.
aloha,
g