To be fair, Serena's growth as a trainer wasn't really the point of her individual character arc, her growth as a person was. Not that the two are mutually exclusive by any means. It's just that with Serena, the latter seems to have been given the greater focus, especially compared to characters past. Dawn, for example, had problems with things like overconfidence that were addressed in her series, and while they occasionally spilled over into her personal life (the depression arc, for example), for the most part they were explored in the capacity of how they effected her as a trainer/coordinator. With Serena, the development she underwent was explored more along the lines of how it effected her as a person/human being in general. Which, to me, is an interesting and slightly refreshing angle to take with a character. Could it have been pulled off better? Yes, it could have. Would I want this to be the focus for every character? No, for the same reason I found it refreshing with Serena: a little variety never hurt anyone.
Come to think of it, personal rather than professional improvement is a theme used with all the XY main cast. Clemont went on a journey not to become an electric type master but to become braver and stronger and experience more of what life had to offer than he would if he stayed shut up in his tower. Bonnie learned through her interactions with Pokemon like Flabebe, Tyrunt, and Squishy what it meant to truly care for and do right by them, and that sometimes that means saying goodbye. Even Ash's character development for the series was less about gaining a special power as it was about his learning to handle that power (and the ego boost that eventually came with it).
