with all this debate of how one becomes a champion of a region.
I assume Ash is the Alola Champion, and not just a league winner, since the Alola region had not established a proper elite four, and the strongest trainer was Kuiki, who was considered the strongest trainer, which is essentially like the regional champ of the Alola region.
The anime is still very vague about explaining how pokemon trainers become a pokemon master, but I guess I'd assume that in order to be a pokemon master, a trainer must win 8 regional leagues and then battle and defeat an elite group of pokemon masters, not speaking of the elite four, to be given the title of being a pokemon master.
A second option to becoming a pokemon master i would imagine is that a trainer must win a league tournament, challenge and win the champion league, and then defeat the elite four and the current regional champion, which would allow a trainer the right to face the elite group of pokemon masters in order to be considered a pokemon master if the trainer can defeat them.
A third alternative option i would imagine for a trainer to become a pokemon master, besides winning 8 league tournaments, could be to become a World Champion, which I assume would give a trainer the prequistes to challenge the elite group of pokemon masters in order to attain the title of Pokemon Master. Since being a world champion puts a trainer at the highest rank of achievement, second only to the title of pokemon master, I'd assume the rank of world champion would automatically grant a trainer the right to challenge the elite group of pokemon masters to earn the title of pokemon master themselves.
These are just my speculations. I truely believe that Iris herself has become a champion, especially seeing how her current team feels far too strong to be gym leader level. Its been a while since she appeared, and i'd assume her skillls have advanced quite alot than just being considered a mere gym leader.
The way there promoting this battle with the stake of hyper class on the line, it seems heavily implied that Ash win be the victor. If not the victor the battle could end in a draw. Though I feel the former my be more likely to happen than the latter speculation.