My mom bought a huge 6" foam piece (can cut that to size, but they usually come in mattress sizes) and a pillow top mattress pad (that fits over the foam) and it makes the sofa bed crazy comfortable!

Yes, I added a memory foam mattress topper to mine and it has made it very comfortable. You can buy these in many stores such as Bed, bath& beyond and even homegoods. Of course it helps if the mattress underneath is in good condition! If yours is not you may want to replace that too! Look on line for sources!