Your first paragraph means that Ash beating Cynthia and Iris doing better against Cynthia than Alain against Leon (probably) won’t make any sense eitherThis would just be poor writing.
The Anime story revolves around Ash and the primary focus is about him reaching Leon.
Right before the Master Class qualification match, we see Ash Greninja effortlessly toy around with one of Ash's ace. Subsequently, in the master class tournament, we see Leon's Charizard effortlessly destroy Alain's Charizard, a pokemon Ash Greninja could never defeat. Plus, Alain's Charizard unlike AG, was busy taking part in the PWC versus cutting weed.
As of now, Ash simply doesn't have the credibility to come close to being any form of a threat to Leon, based on all of his performances so far. While the win against Steven would definitely help, it would be ideal for Ash to come across one more trainer of similar monstrous strength before the Leon fight, and the perfect person for this would be Cynthia given that she's ranked 2, the anime's most established regional champion with the longest standing relationship with the MC.
Iris beating or even coming close to Cynthia would just be terrible narratively simply because of her previous performance against Ash in the Great Class. If Iris effortlessly destroyed Ash in that battle, it would've been a different story. However, that didn't happen. In fact, the performance there was just so pathetic such that Pokemon's already inconsistent power scaling was completely thrown out of the door while having her lose against pokemon like Ash's Dracovish and Dragonite.
If they wanted to give Iris her moment, they should've matched her against Lance or Diantha. Matching her against the perfect semi-final candidate for Ash was a big mistake.
Ash definitely needs a high stake semi-final combined with the Steven match to make him ready for a fight against Leon (The strongest trainer in the world). Iris actually forcing Cynthia to go all out or even beating her would absolutely lower all the stakes and more importantly, take away any credibility of Ash being any real threat to Leon.
I think the perfect match would be Cynthia mostly mentoring Iris throughout the match and focusing on her growth since the last time they fought but more importantly, Iris realising there is still a lot of room to grow/great pinnacle of strength to reach for her to become the Dragon Master (And have the strongest Dragon owned Pokemon).
This can be established by Cynthia Mega Evolving Garchomp to reveal to Iris what her being serious or going all out is really like.
Now the only rational option to force Cynthia to get serious and go all out, would be to have Iris come in with a unique ace. Something like Zekrom/Reshiram or maybe even Mega Latias/Latios (Outside of Haxorus that lost against Ash previous) and have Cynthia defeat it in a serious fight.
Again I repeat, the anime is about Ash and not Iris. The former is the MC and the PWC arc should focus on him just like how the focus of Project Mew should be on Goh and not Gary.
Leon's dominance over Alain indicates how powerful Leon really is. As of now, from a plot perspective, there just isn't enough narrative to support Ash being a worthy enough challenger to Leon if they battle.Your first paragraph means that Ash beating Cynthia and Iris doing better against Cynthia than Alain against Leon (probably) won’t make any sense either