It's all a matter of preference or taste.
As I mentioned on the Rafferty thread, where you're not allowed to be even slightly negative , music is very subjective.
It cant be measured by record sales as the OP has mentioned some "artists" have sold bucketfuls of copies and would generally be seen as "throw away" attempts at music, at best, but millions have liked them enough to go out and purchase them.
No, I don't believe there is definitive "bad music", just different taste.