… You can probably just pair a Joycon 2 with the deck to get the functionality. Since the steamdeck does not feature detachable controllers in its design, an external controller is needed. Any controller that connects via Bluetooth or other wireless standard pretty much works - Bluetooth as a standard even allows a single device to use different profiles.

If the feature for “mouse mode” is implemented in the Joycon 2 hardware, there should be little effort needed to make it work with the deck. If it is a proprietary host/device driver interaction, it probably uses standard HID signals because those would take a good amount of time and money to reinvent, so, reverse engineering everything needed for the driver should be fairly quick and easy for people who deal with Linux device drivers.

Personally, I tend to just use the trackpad on the Dualsense controller as a mouse. It works, no bother.