Is light good or bad for brake fluid? You see many clear reservoirs but they usually fade opaque.
On a sportbike they are clear but that could be to help keep it in check. But some motorcycles use the metal fluid container.
Even if you expose the master cylinder to light with the hood open, how often is that?
The light's always on in your refrigerator, too.
My Virago has a 1 cm round glass window in the master cylinder, FWIW.