56% winrate with the 2nd highest pickrate (less than 1% behind thresh). She's been this way since basically her release and all of the love tap nerfs to her don't really change anything about how good her kit is at peeling and reseting fights. I'm awful at supporting, but if I get autofilled to support I just pick Janna and watch as my ADC never dies even if they are trying to intentionally feed. It's very frustrating playing a melee champion and trying to be of any use at all to your team because the enemy team has a Janna that just screws you over all game long.
She has a ton of power in her kit, but it's not obvious that's it there which is why she doesn't seem overpowered in game, but, statistically, she is completely ridiculous atm and has been that way for a long time.