I think banrates are a bigger indication if a champ is OP, in general
If a champ is good, people are going to play it even if its boring to play. That's just how people on league are. It's fun to win, regardless of how you do it. Look at some dumb strats like int Sion, I can't imagine how that's fun but if it wins games, people will do it to finally get that gold skin or whatever.
If people play good champs, and thus people lose to that champ, they will consider banning it more and more.
When I look at champs with extremely low banrates, it makes me wonder. No one ever gets smashed by this shit to the point where they consider banning it, really?
Of course generalizations are never 100% and there are counter examples, but I still think there is a stronger correlation compared to win rates.
I think pick and ban rates are more important.