Gaming/streaming using Mirror (1.1.0) doesn’t prevent the Mac screen saver if using the Playdate as the controller: those inputs don’t interrupt the system screen saver timer the way keypresses (and hopefully gamepad inputs?) do.
If there’s no way to tell the system about that kind of input, then maybe there could be a setting to just disable the screensaver automatically (or at least in fullscreen mode) the way video playback does?