The switch should have no problem with 1080p, it is using the Tegra X1 chip, which is capable of 4k at 60fps. Using 1/4 or an 1/8th of the GPU capability would not be causing overheating. I suspect something else, Nintendo released a patch a few days ago which seemed to fix frame rates dropping below 20, the details of the patch were "Adjustments have been made to make for a more pleasant gaming experience." so I can only speculate. My guess is that there were severe inefficiencies in how the GPU was rendering which the patch fixed causing the GPU to run much cooler.