Joy Malignant is playable on the Steam Deck!
Controls
Since Joy Malignant is entirely playable using only a mouse, the game is intuitively playable start to finish on the Steam Deck using the trackpad.
Controller support without usage of the trackpad is not currently in place, but might come post-launch if sufficient demand is there.
Performance
Performance on steam deck is identical to most PCs, as the game is not graphically demanding.