It’s definitely not as nice as the Steam Deck because the Deck has touchpads. The Ally has a mode where the controller can act as a mouse but it’s not great, and it requires the ROG software to switch between mouse and gamepad modes. Otherwise you’re stuck with the touchscreen. I would recommend mouse and keyboard if you’re going to be doing anything other than running the SteamOS/Big Picture UI (or the ROG Ally UI, but it’s not great).

On Linux, I made a program that can use a touchscreen as a mouse that should be able to work on any device with a touchscreen. I originally made it for the PinePhone so that I could click on things easier, but with a few tweaks it would probably work on the Ally for a pseudo-touchpad.