I guess it depends on the topper, stayed at my parents shortly after surgery to my collarbone. I could only sleep on my back due to the surgery, they had some sort of soft topper which they said was really comfortable. I woke up at 3 in the morning with my back in agony and had to sleep sat up for the rest of the night. Moved the next day to another house with a comfortable sofa bed.
Don’t really see the point of them anyhows, unless you’re trying to raise the height of the bed. wouldn’t you be better off just buying a good mattress?