I think the writers don't want Serena to be that dynamic because she'll be too much like Dawn then. The writers strapped themselves into a corner here.
If she's too energetic and dynamic she'll be like Dawn, if she's too well characterised and bubbly she'll be like May, and she's designed to be as opposite as Misty/Iris so she has no temper and doesn't mock Ash. So Serena has no choice to be as she is.
You act as if all the Pokegirls are just clones of one another with no defining characterization of their own. Could Dawn, May, Serena or Misty scale a tree in mere seconds like Iris? Could any of the Pokegirls beat May in an eating competition? Do any of the girls have a crush on Ash besides Serena? The girls share traits that are similar to one another but that hardly makes them too similar for their own good. Iris and Misty are a bit snarky, you know who else was too? Max, the little boy Ash traveled with. Misty
loved fashion though, Iris found it vain and boring however. Misty's goal got little to no exposition, Iris's did. Misty owned a jillion Water-types and was a Gym Leader, Iris was not and only had two Dragon-type Pokemon under her care.
Dawn loved flashy/shiny combinations, May preferred straight up attacking and causing pain while looking fierce with her combinations. May did regular Contest, Dawn did Super Contest. May never went through some spirit-breaking arc. of sadness over continued Contest failures. May was more laidback and showed some tendency of liking cute things, like imagining herself as a nurse, liking flowers(OH WAIT, IRIS LOVE FLOWERS TOO AND CLAIR'S DRAGONITE LIKES FLOWERS CLONES!!!!111) or catching an Azurill but she also was very plucky at times. I don't remember Dawn getting as terribly embarrassed by her rival and it being a massive cause for concern either with her unrelentlessly attacking and her not being focused enough to battle correctly or Ursula being as much a villain as Harley or even a villain at all. Ursula was more a
bitch than a villain Coordinator like Harley. Zoey and Drew are world's different, May's Pokemon and style were vastly different than Dawn's. May's appeals seemed more simpler in the sense they felt more sharp-paced, quicker and less gaudy than Dawn's more sparkly, pretty stuff. May would teach Skitty how to juggle or throw flying disk as an appeal at times.
In real life some girls share similar emotions(All people do, boys, girls, men, woman, babies) and all but by no means does that make
them clones of each other because they're different at the end of the day at the heart of who they are.
And really? Dawn quit being
dynamic after early DP. There was no going off on Paul for being a jerk(She got up in his face for forgetting her name as well as his behavior at the Tag Battle), there were no sass jabs at Ash like what we saw in the Tag Battle episode, there were no amusing moments with her mom after that point, Johanna became boring as sin after that point in the series and rarely was the scolding pageant mom we saw in early DP. Otherwise, Dawn, she became more girl Ash-like after a certain point in DP. Piplup grew into an out-of-the-ball Pokemon, it wasn't one off the bat and even then Serena doesn't have one of those, Bonnie does. Quit trying to claim Serena to be a Dawn clone, you're the only one doing this regularly who I've come across on any Pokemon site.
Because at the end of the day the Pokegirls aren't
that freaking hard to differentiate for anyone with eight or more chromosomes running through their head, oh my God, a five-year-old could be more perceptive with a crayon and Venn Diagram.