I don’t think Paperpile needs to add a ratings feature. Stars are probably good enough for most people. However, if you need ratings, you can create a folder called “ratings” and then create subfolders called “★” “★★” “★★★” “★★★★” and “★★★★★”.
The only problem I’ve had with using folders for ratings or status indicators is that when you move something to a new folder, it doesn’t get moved out of the old folder. It seems Paperpile folders are more like labels in this regard. You need to go into the menu at the top and uncheck the previous folder to remove it from the previous folder.