Same question here. I want to play StS from my couch, but it's inconvenient to have to run upstairs to launch it from my PC.

If you're playing via SteamLink, you can hold the back/select button to pop up a window that lets you toggle mouse mode on your controller. Use that to clck through the window, then switch back to controller mode.

Not on steamlink...
but I know you can hold the PS logo or Xbox logo on the controller. It will let you move the mouse and I believe it's right bumper to click the mouse. Note: if you hold the logos too long the controller will turn off.
Still inconvenient but works in a pinch.