To cut hay properly you need a disc, drum or fingerbar mower. These cut the grass cleanly so the stalks of grass are left whole and undamaged.
A topper has a big (blunt) rotating blade that smashes up the grass (great for when you are leaving the cuttings lying as they break down quicker, and for cutting coarser things like thistles). I don't know if you could make hay from the resulting grass but I doubt it would be easy or much good.