It...it just had a dream.
Why are you making up these rules that Iris should have followed to be a good character in your eyes? LOL
The lone fact that you are comparing her goal to Dawn or Serena's goal shows how biased you are.
You do remember that Iris's original quest as handed to her by the village elder was to go on a quest and raise Axew into a strong Haxorus, right? So in that sense, Iris never really grew in that challenge at all since Axew barely grew. Yes, by the end it wasn't the timid baby or weakling it was originally, but a lot of the training was wasted. For example, Axew learns Outrage miraculously since the writers needed a way to get Axew a win against Luke. They never use the move again (unless my memory is off. If so, I apologize). Then, against Cynthia it once more learns a powerful move in Giga Impact only to then not use it again or even train the move. So, you can't say she really progressed in her original goal.
However, Iris did improve over time as a Dragon Master as shown by bonding with Dragonite and being able to help it and then calm it down. Not only that, but she managed to get Dragonite to trust her.
Though even with that, one cannot argue that the plot bended to allow Iris an easier time. Examples would include the fact that Excadrill was revealed to be disobedient and yet it had no build up to becoming loyal and overcoming his depressed state. It just happened a few appearances later (all of which he would just lay there doing nothing unless attacked first) and after losing a battle with Georgia they became partners again. Now, they could have done something similar to Charizard where Ash actually tried to use him over and over in battle to get him to listen and even then it took him staying up all night keeping Charizard warm for him to listen to Ash.
Another example was mentioned above with Axew in the case of him always learning a new, powerful move just to do well in a battle. Granted that he had no shot against Cynthia, but it still learned Giga Impact with no reason.
Next would be Emolga. The electric type never developed from her capture to the end of the series. It stayed the same way always trying to manipulate a situation to better herself. Yes, she looked out for Axew, but she also stayed lazy and gave Iris hard times.
So, in sense, Iris wasn't the worst developed but she wasn't the greatest either. She had a lot of defining moments but also had a lot of moments given to her without any complaints.
Either way, Iris is a highly divisive character where some will argue she developed well and others won't. But, objectively speaking you can say she was average compared to others. She developed better than AG/DP Brock, Max, Cilan, Tracey and Misty.
