To implement the thermal design I might put on a heater or a radiator or a blanket of some kind. I do my analysis by computer modeling. Based on the environment, whether the spacecraft is pointing at the sun or not, I make a computer model of the payload. All disciplines are important on the design of the spacecraft. We're all kind of linked, all equally important, all pieces to one giant puzzle.
Thermal design is important.
If things are not maintained within certain temperatures, they will fail. For example, the latest theory of why the Galileo high gain antenna didn't deploy is because of thermal expansion at an interface that was not designed properly. Two pieces expanded and got stuck. Another example is on the Magellan mission. If I don't do my job right, the mission will fail.