Michael Jackson - Benefited from technology, when technology was starting to boom. Of course people will be attracted to MJ with the technology that he was using at that time.
Thriller overrated, very overrated, because HIStory is the best album he's ever made

This album showed his true artistic value.
Madonna benefited from shock value, whatever hasn't been done, she was the first one to do it, which is why you see Lady Gaga, Beyonce, Katy, Rihanna, etc.. doing all these crazy stints, and the GP is like "this has been done a million times already", meaning Madonna has already done these. It's like filling an empty void, and now that the void has been filled, there is no longer excitement.
None of them are overrated though, they've both sold 200M albums WW.
Now, if we're really talking about overrated, we'll discuss it over a cup of tea.