I don't think Iris is undeserving of being Champion, she's a strong trainer, and she earned her title by defeating Alder in battle, something Drayden mentions in one of the memories that comes with Memory Link. What may seem childish, I see as young and eager. She is definitely not as mature as the Elite 4 or the Gym Leaders, but that doesn't mean she isn't serious about her responsibilities.
However, it really took me by surprise seeing her as the new Champion. I really, really wanted it to be one of the BW protagonists, though I knew it was too much to wish for. In my opinion, and realistically speaking, Cheren would've been the best choice, he was there from start to end in the whole Plasma affair in BW, and he's hard-working and strong. After seeing him as a new Gym Leader, then I thought the new Champion was maybe one the Striaton brothers. It would've provided an answer as to why they weren't Gym Leaders anymore, one of them becoming Champion, while the other two deciding to close the Gym and further hone their skills to reach their brother's achievement.