There is no champion ranking.
The levels in the games are tied to the game only.
What could you expect as levels? If the levels in the DLC were lower than those in the main game would you have been happy? It is normal that Cyrano, Nemona and Kieran have such high levels, but there is no real champion ranking of all champions.
It is a technical thing related to the game. Nemona is stronger than Alisma, and Keiran is a very strong coach (but he is not a champion, as he is not a true champion like Nemona or Geeta, but he is the champion of the League Club of the Blueberry Academy, an internal system that applies only to them, and they are therefore considered very strong coaches, but he is not considered a true champion, just as Drayton, Lacey, Crispin and Amarys are not really Elite Four members like Aaron, Bertha, Flint or Lucian).
Nemona is the strongest champion, that I think is confirmable.
But if Nemona's team was already high, the final boss of the street of fights had to be one of the strongest coaches in the game as levels, he could not have lower levels, also because the player should have naturally arrived at those levels there at that time in the post-game.
Just as Penny is the strongest team leader there is, just as Volo is the strongest villain there is.
Just as there was Mega Rayquaza surpassing everybody, just as there was Necrozma surpassing everybody else....
