Any form of exercise, including cycling, can be incredibly beneficial for asthma sufferers.
That being said, that doesn't mean that you shouldn't take precautions. It's an unfortunate fact that while exercising, you definitely run at an increased risk of triggering your asthma. For this reason it's important to always have your inhaler with you.
Keeping that in mind, exercise can help generally reduce the symptoms of asthma. Aerobic exercises such as cycling help strengthen the lungs and improve oxygen intake even when you're not exercising, which is obviously beneficial for anyone, but exponentially so for asthma sufferers.
The rewards definitely outweigh the risks, so get out there and do some cycling (with your inhaler at hand, of course).