http://champions.pickban.com/l/86/Sona
this is for Diamond-CHallenger, she is sitting at almost 58% actually (57.8%) with a 12.3% pick rate, soooo yeah thats pretty high.
i only use pickban to look at the strength of champs but you can also check out other sites like i know op.gg is a popular one but the difference is that a lot of other sites like this use platinum to challenger which can sometimes be very different than if you look at the diamond+ but a strong champ will usually show high winrate regardless
p.s. she has atm no bad matchups according to the data which i haven't seen on this site before, literally she has a 56%+ winrate vs any support and she has strong winrates with virtually every adc..yeah she is gonna be gutted hard if the data continues like it is
p.p.s for list of full champions since 6.14 dropped http://champions.pickban.com/, get to see how champs are doing so far at the highest level