Win rate alone means very little (except if it's way off charts ofc, anyone with a 75% win rate for instance is clearly overtuned), but combined with pick rate and ban rate, now you have something.
If a champion has a very high ban rate, is picked nearly every time he's left open, and has a very high win rate to boot, it's probably time to take a look at said champion balance-wise.
Low pick rate, low ban rate and high win rate means the champion has a niche he's picked for and does well in said niche, therefore not automatically too strong.
On the opposite, a win rate slightly below 50% does not automatically mean the champion isn't too strong; if said champion has a high play rate too, then it is likely people who are inexperienced with him are playing him a lot too and lowering his win rate.
You also have to look at the game as a whole, sometimes items and other external factors are to blame more than the champion. This is very true for bot laners where champion synergy is important; Sona+MF currently has a high win rate, because Sona+MF has always had good synergy, and MF is currently very strong. Does Sona need nerfs atm? Unlikely. MF? Most likely.