Recently,while watching the Pokémon X and Y anime,I noticed Serena had a strange characteristic...Upon entering an old spooky mansion,She begins saying the worst case scenario possible,then says stuff like "Oops,Did I say that out loud!?"I haven't seen many episodes of X and Y,Does she do this often?And if so,why,is there an explanation given?
I've been following the anime for some time(don't judge) and that thing where she says oops is NOT a regular thing with her. If I where to say anything it was so that that specific episode could have some added humor with serena's clumsy behavior.
So to basically sum it up, no Serena being a loudmouth is not a part of her personality. She has never said it other than that one episode, unilke Iris calling ash a kid like every episode.
Well hope that helps you.