It wouldn't be great. The goal of DLSS3 and very likely FSR3 is to boost fps from 60 to over 100. The more under 60 you go the worse the image quality and motion gets. 30->60 would not be great. It could work well for the ROG Ally though due to the 120hz screen.
This. Since the Steam Deck's display can only handle 60hz you'd have only 30fps really to play with FSR 3 - assuming it works with RDNA 2. And with the added input lag when using frame insertion, it would be horrible to play. Better off limiting fps/hz to 30 or 40 and leave it be.
My personal experience with DLSS 3 and frame gen is that really for a passable experience you'd want at least 40fps (80fps FG) baseline to be the absolute bare minimum (and thats with Nvidia's Reflex) and 60fps+ (120fps FG) to be a good experience. With FSR 3 we have no idea but I'm going to guess its not going to as good as DLSS 3, just yet. We will see soon, hopefully.. if rumors that its coming next month is true.