There is no justification really to change Iris and Cilan characterization in any way. Let alone just because XY series have different writing style apparently requiring that others from past get subjected to current atmosphrre and "allegedly more mature vibe new saga radiate with".
Unless behind changes in way they behave or bact is provided somesort of background, explanation through specials or flashbacks happening prior to that or simultanelusly with it which would shed light on certain deviation from usual demanor. And what caused or prompted Cilan and Iris to change happening as result of character development. Which requires set up, several episodes and external factors introduced in character story like it was case with first and only set of chronicles. Somesort of buid up(wont happen obviously).
Ash acted more mature in Hoenn but did this stopped Max from treasing him, May getting into clash or Misty in her cameos still being annoyed by his behavior teasing him out of endeavor coming with sarcastic remarks?
Same applies to Sinnoh and several clashes Ash experinced with various character be it Paul, Angie, Barry etc?
Just because Ash acts more mature and anime is in certain people eyes containg more mature plot doesn't and shouldn't decide how much comedy, clash and drama is allowed for character dynamic and anime to have.
On contrair in every storyline, no matter how lighthearted or gloomy it is humor, wacky antics and lighthearted jokes,pranks and tantrums happening between characters occasionally are desierable and needed to lift up surrounding and bring more energy in interactions, while building on relationship.
So Cilan flamboyant and patient personality being curious and adept in many professions along with Iris hyperactive, wild, snarky attitude would work within Ash, Serena, Clemont and Bonnie characterization just as much like it was case in Unova.
Or even better because collide of different personalities and views on life only borns new, unexpected and different kind of relationship between characters making them more glaring and packed with spice.
Well they did kinda change Dawn's personality in BW, so why not? In BW Dawn picked up Iris' "what a kid phrase" and started mocking Ash with it. Back in DP she never did this.
I sometimes wonder if the writers had Dawn start to do that because they already knew they were bringing in Serena at the time, so they made Dawn "distance" herself from Ash so they could bring in Serena who had an obvious crush on Ash. The writers probably wanted to make sure everyone knew Dawn didn't have a crush on Ash, so it would be more obvious when Serena came in.
From wherever some people got impression that "Dawn was entirely different character during BW cameo" it simply wasnt true. I would never imagine that Dawn finding Iris catchphrase funny deciding to play along with her would cause people pulling out of context act of teasing someone out of endearment, pertaining it to malicious intent of downplaying Ash.
Dawn was on good terms with Ash just as much like she was in Sinnoh, She eagerly awated for Ash arrival in Cynthia mansion, decided to surprise him with her visit seeming very eager when they met each other again. Talked about her interest in pokemon musicals and super model Elesa along with having battle against Ash for old time sake between Quillava and Pikachu only further strengthening friendship existing between them.
Dawn was still girly and optimistic, using her catchphrase "no need to worry". Concerned about her appearance trying to fix hair, cheerful and interested in fashion.
And yes teasing Ash or sometime being whimsical, agitated and angry wasn't something which magically appeared when meeting up with Iris. Dawn got several times in chash or with Ash and others like Barry or Paul, got irritated by others or joked like Conway or Lyra, got annoyed with Ash habits and behavior etc.
Happening long before BW even started.
I found BW to be more kiddish than DP regardless of how temporarily serious TR were.
