If you say her development did not stop after Wallace Cup, then tell me, what development she actually got? New Pokémon? Pokémon who did not listen her? Oh, come on, many characters got that. And working on her friendship with Ash? If that is sign of character development, then Brock should be the most developed character after all the time he spend with Ash {XD}
And no, being prepared is not crime. But it really would not leave much space for character development and Dawn is, like it or not, great example of that. However, Clemont actually show us that even skilled trainer may still get a lot of character development. To be honest, I just think back in DP, writers simply did not know how to do character development, so they did BW, whose characters did not really need any character development, to let them think how to make characters with some real character development, which they finally got in XY. In XY, all the main characters, except Team Rocket trio, unfortunately, got some character development, with still leaving place for future character development. You say Serena still acts like novice, but that is actually good thing, since we are, most likely, somewhere in the middle of her stay in the show and there still is way to go with her character. And that is exactly what this show needs, after all those stalled characters in past.