Popularity isn't proof that someone is a good writer, but it certainly is evidence of same.
Only a small percentage of people buy books regularly. So to make a big bestseller you have to write a book that gets bought by that huge majority of people who don't read much. And usually their decisions are dominated by random extraneous factors: i.e. it's a biography of someone they admire, or is related to some doctrine they subscribe to, or contains facts they need or gossip they want to know about.
Think of the books you find in the houses of people who have hardly any books: the Bible, the Guinness Book of World Records, a Chilton car repair manual, a racy biography of Princess Di, a book of popular mysticism, a book about how to make money or lose weight. Those are the really big sellers.
(This question sounds smarmy, but I ask it earnestly.)
Good judges can tell who the good writers are, but there's no currently no independent way of telling who the good judges are.
Empirically the oscillations of opinion seem to settle down after a few hundred years. Even then there are probably errors. E.g. tragedies will tend to be more highly regarded than comedies, because they seem more serious.