This seems to have been resolved and Love, Ghostie now launches properly on Proton 9.0-2. However, if at any time you run into issues on Steam Deck, you can always try a different Proton version to see if that works in the meantime!
A new Proton version was released recently on Steam Deck (9.0-2) and unfortunately the Love, Ghostie demo crashes on that version. I've filed a bug report with Proton, and they're looking into a fix.
In the meantime, you can switch your Proton version to 8.0-5 or 7.0-6 to get it working!
How to change Proton versions on the Steam Deck
Hoping that it gets resolved soon!