There's a button combination that flips the screen and lets you write on the touchscreen for games like Mario Paint. Check the documentation for the button combo, it's something weird like L+R+select-up. Use a clean rom, not one that's been patched for controller button usage vs the mouse. It also might require a specific version of SNEmulDS, not sure if the newer versions ever kept compatibility with SNES Mouse mode or not.
I demonstrated it in one of my old video demos, skip to the very last minute for the relevant Mario Paint demo.