It’s in fact an option not a shader.

Go in Retroarch Menu of PSX emulator and then choose Core Options.

You’ll find an option to get higher resolution but performance will drop a lot.

Btw, you are able to control Mario in Super Mario all stars ?? I tought it was not possible to run (control Mario in-game) this one in Retroarch…

Btw 2, your graphics seem so great for Snes ! Far better than mine if I’m plugged on HDTV.