From what Riot has said Jinx has always had a high percentage of "Mains."
But your comment about it being a third of the playerbase is absurd.
There are 10 players per match
a 34% pick rate means that Jinx appears in 34% of all matches.
so in 10 matches, where 100 champs total are selected. Jinx appears 3 times.
She's probably mained by no more than 3% of the player base.
Worse you are another player who doesn't get what it means to "main" an ADC.
Now I'm in silver, and I'll admit I have no idea what happens in the plat/diamond ranks, but remember that more than half of the player base is silver or bronze, so what happens down in those ranks affects the pick % and winrate.
In those ranks ADC is NOT mained by many players. In fact in the ranked games I've played I've had an ADC main on my side once. Everyone seems to fight for top/mid/jungle, and then aDC and Support are left to whoever is left. And most of the time you DON'T get Jinx picked in those games. People who don't main Jinx don't appear to pick her as often. Part I'm sure is her cost, she is 6300 IP and that's a lot for someone still getting runes and champs if they aren't going to main her.
So when you do run into a Jinx in bronze/silver ranked play it almost always is someone who has played the champ many times. And experience = wins. That is what Riot has stated.
Jinx is not broken. I've played her and I've played against her and wrecked her. There are some easy ways to defeat a Jinx player, and I've seen all of them used. Its only when you start obsessing that she's OP that she can destroy you again, and again.
My advice if you have that much of a problem defeating Jinx, either learn to play her or stop playing completely. She's not unbeatable if you take the time to learn how.