Even so, the character development may got in AG was commendable. Her contest arc greatly differed from Dawn's contest arc in a sense that rather developing May's team, they developed her personality more. I mean, May was more or less stuck with Beautifly, Combusken and Skitty while Bulbasaur joined the fray in the middle. Her contest arc always associated with a bit of melodrama like Harley humiliating her publicly, her own confidence issues etc etc. And not to mention May's name almost always ended up in the eighth place after the appeal round.....she got her 5th ribbon in the last contest of Hoenn. She messed up pretty bad in the Grand festival thanks to Harley and barely made it to the battle round. All of these pointed to the same thing: It was May's character that was mostly tested more than her battling skills. IMO, her battling skill wasn't even talked about until the last episode where she loses to Solidad.
Dawn on the hand was written completely different. While her arc was filled with quite a bit of drama too, her troubles associated with her skill as a coordinator rather than her character. She didn't had any trouble with people like Harley. Even Ursula only insulted her but didn't go to extreme means to stop her progress. Dawn's development associated with 1) Her realizing that she needs to showcase her Pokemon not the flashy moves 2) The combination moves she worked to beat Ursula etc. Dawn's contest arc was written quite differently because the writers obviously didn't want her to be a rehash of May. While she did have some sad moments, there wasn't a constant drama revolving around her like it did with May. She passed the Contest Appeal with flying colors in the grand festival, she got her 5th ribbon well in advance etc. etc.
Now those who like Dawn or May usually tend to differ because of their preference towards the storylines. Those who liked May's drama-esque story line preferred May over Dawn and disliked her while people(like me) preferred the second option over the first. It's a matter of opinion.
But one must agree that almost all of Dawn's character development was in fact "battle" development. It's not a wrong thing, it's just that the writers wrote her that way. There are many people who enjoyed Dawn over May. In the end, it's a matter of preference.
Iris, imo comes with character development. Her battle skills were never truly tested(except by Drayden). Her only "battle" related development was teaching Excadrill Focus Blast. Most of her development comes under character development. I think Doryuzu, being a Iris fan, has done a good job explaining Iris in their post if you check the previous page.