why is a 0 skill champion "janna" the best champion in the game

The average rank of Janna mains is around gold 5, and since the majority of the game are bronze and silver, this actually does mean that there are a lot of Jannas in high elo.

To put that in perspective, only GP, Taliyah, Ivern, TF, Rek, Sol and Elise have an average rank of gold+ with Ivern and Sol sitting at Gold 4. She also has the second highest main rate among those champs.

Of course there's no way to prove that Janna isnt just simply a better pick in high elo but those are the stats, according the leagueofgraphs.