No, it's not sweet and edible. Round, though, with a hole in it. The helmet donut gives your helmet a fixed location and prevents it from rolling around when you clean it or change its vizor.
A good helmet is made to protect from a fall once and it's a shame if that happens when it slips out of your hands or rolls off the table. Hence: the helmet donut. Less tasty than a regular donut, but you will enjoy it for longer.