The default Ubuntu image for Jetson Nano 2GB uses LXDE as its desktop environment, because LXDE should be light-weight enough. However I’ve noticed that even LXDE periodically occupies CPU for nearly 100% thus causing a higher temperature.
Just tried Awesome wm (https://awesomewm.org/) and it has even less footprint than LXDE - never saw 100% CPU occupation by X server process. Though it may be an over simplified desktop environment but it does save extra energy and unnecessary workload of my board.