According to RIITO, you are wrong. His win rate via mastery is actually not as good as you would expect with mastery of the champion. He isn't rewarding his one tricks almost at all with a higher win rate.
"Highest winrates" in just diamond doesn't mean much when it's only right around 50%. Yasuo struggles to find a place in high ELO lay. He's a bigger problem in low ELO where people are (generally) less skilled and less coordinated.
He needs some work, but not because he is too strong. He's just REALLY popular and somewhat problematic for the majority of the player base. Riot would love to see this champ in LCS too, since he is one of the most popular champions in league atm.