I don't think it would be. It'd have to have an upwards buoyant force of most of your body weight to really be effective. At that point we are talking about a huge balloon, meaning it'll cause a lot of air resistance which will cause problems when you're attempting to ride. You'll also lose a lot of traction. I think in the end it'd make it easier to be tipped over (especially if it catches a wind gust), and so would be self-defeating.