Up to version 5.15 you could simply plug in a mouse + keyboard and while in a gamecube game, just press ALT + TAB, to go to the menu of Dolphin. There you can navigate via mouse / keyboard to the graphics settings.
But in version 5.16 this does not work anymore. You can get to the Dolphin menu with ALT + TAB, but then no navigation via mouse/keyboard is possible anymore 🙁