I'm actually of two minds about it, now that everything is said and done.

I'm not the biggest fan of Iris going all the way from okay trainer that could become a Gym Leader to the Champion of the Unova region in 'two regions' time, since to me it felt like a move made because she was the latest Unovan Champion in the games rather than because it was where her arc was heading, like a lot of JN's most 'commercial' decisions. I also wasn't a fan of how the battle itself went, both for awkward pacing and because I believe having a newly born Pokémon that Ash admitted could only use Water Gun mere episodes before defeat Dragonite easily kinda undermined the idea she's grown stronger. So, on the whole, I'm not the biggest fan of her reaching the Masters Eight since it felt like result of the show trying to promote the Masters Tournament as a 'Tournament of Champions' plus Ash and Alain (and relatedly, I recall someone also pointing out logically speaking, she couldn't have reached the Master Class before Ash given he entered Ultra first, all matches are scheduled, and Ash lost none of his).

However, in spite of that, I actually think Iris VS Cynthia is a very good battle: it actually pays off Cynthia and Iris's relationship in BW, is a nicely character-driven fight compared to all the other matches in the quarterfinals, and aside from a few JN-typical issues of some Pokémon ending up way too frail and going down too quickly, it's a fairly fun brawl. Even if Iris didn't win I believe that battle is a much more fitting capoff to her arc than her battle with Ash, and I'm glad we got it in spite of my grievances.