Oh boy my favorite subject.
Overall, as mentioned, Ash has grown as a character. But his development seems to stop at Sinnoh, after he lost against the league. However, I think the move from Sinnoh -> Unova -> Kalos has been a different kind of development. Perhaps not in the strength deparment, but in other aspects.
Unova you see him basically reverting back to older times, so to say. Forgetting what gender his Pokemon are, trying to use attract on the wrong Pokemon, so on and so forth (there was also the disaster of Elesa's gym but let's not go there). But there were times when he was showing that he has grown a lot as a trainer as well. Teaching the Cottonee how to fight in order to get a girlfriend, and the lessons that he teaches Cilan and Iris unintentionally as well. I could go more into detail, but my thoughts on him going from a powerhouse of sinnoh to what he was in unova a psychological deal.
After his set back of the Unova league, though, he comes to Kalos and becomes more of a powerhouse. More like Sinnoh. For quite a while (between Korrina and the Ash-Greninja stuff) Ash was pretty much on a winning streak. It was getting so long that I was starting to feel like that he needed a few losses, because I don't think his development is done yet. I don't think he should be that unbeatable trainer yet. Thankfully, XYZ has given us a few losses, and development on the side.
That's just his strength as a trainer though. When it comes to the other sides of him (such as his attitude), the change between Unova and Kalos is pretty easy to explain. For the most part anyways. Don't ask me why there hasn't been as many Pikachu and Ash moments in Kalos because I can't tell you (and something that I really wish they would fix. It's probably my biggest pet peeve, but I love XY anyways so eh), but the dymanics between him and his companions pretty much explain it all.
This is mostly in response to what Famon said about Ash's maturity. Back in Unova you had Cilan who was older than him and then Iris who teased him constantly. Ash wasn't the most mature in the group and he had someone who would bring out the immaturity in him. He also had Trip who also pushed his buttons in more 'childish ways'. But there is no one to do that in Kalos. Instead you have Sawyer who likes Ash and roots for him (quite a difference from Paul and Trip), Serena who is head over heels for him, Clemont who is too nice to really pick a fight with Ash, and Bonnie who also wouldn't pick a fight with Ash. That said, Ash is the most "experienced" trainer (I put that lightly because Clemont is a Gym Leader. Unlike Ash though Clemont hadn'tdone much traveling until his travels with Ash), so it puts him in a leader role.
That said we will have to see what comes out of XYZ and the league, and then look back to see how it was at the beginning and the end of Kalos and what he brings to Alola as a result. Whatever he brings would be his development whether it's negative or positive. Whether or not he's "stronger" in one regard but not in another, etc.