Title. Winrate is only ONE of the factors that people should use when factoring into an argument about a champ's balance. There are other statistics as well, such as banrate, winrate with 50+ games on the champ, play rate, as well as eyeing the champion's stats per game. If a champion is constantly high on the damage charts without any actual damage items, then yes, it's fair to take a second look at the champion.
I'm looking at YOU, yuumi players. People keep bringing up her winrate (omergod her winrate is at 30 percent sweet babee jeesus she is weak REEEEE) but fail to take into account that
a. her pick/ban rate still rank top 10 amongst supports
b. her winrate increases significantly for people who've played the champ a large number of times
c. for people who did play yuumi, the average number of games played on her was so low to rank bottom 10 amongst supports.
d. said people are playing her wrong and eating croissants while playing her
e. her dpm and heals per minute are stupidly high - she can deal as much as a carry in teamfights while also healing/shielding more than a sona, nami or janna
f. she's an enchanter that cannot be focused down due to her stupid attach mechanic (seriously, she cancels delayed ccs and targed abilities by just latching onto someone else) and is STUPID when paired with a garen, master yi, or tryndamere