So Serena made a rookie mistake and from then on made sure the clothes were properly outfitted. I don't think outfitting Pokemon takes some great skill tbh. And we're talking about Pokemon here, those creatures that have physics-breaking acrobatics and mobility. Is being cognizant of Serena's slightly longer ribbon while walking too much to expect from Fennekin? As you said, they are a team. The other loss was because Eevee had a character flaw that Serena had to get out of her system by making her do a Performance; there was no way around it. Only the final loss to Aria was she actually outplayed (though really it's because Serena just doesn't have the same degree of popularity yet), but Aria is the equivalent to a Champion; she's not a rival like Shauna, Miette, or Nini.
In none of those losses was a situation where the rivals outperformed Serena either in the Theme or Freestyle without Serena making a mistake. If Fennekin didn't trip, Serena would have probably won that Showcase. If Eevee didn't trip, Serena would have definitely won that Showcase. For example, Jessie had routines where she did everything right, but it just wasn't good enough because......reasons.
Take Ash vs Tyson or Ash vs Alain. Ash just about battled as well as he possibly could with his regional team, but it wasn't good enough against those guys. Serena's rivals weren't compelling because there was never a moment where Serena checked off the boxes but came up short against them.
Again, that is a problem that you have with the concept of Showcases. It's entirely subjective and has nothing to do with the actual rivals.
Personally, i do think that it takes skills to do any of those activities, whether it's dancing, coreographing, styling, sewing, modeling, cooking or anything else that performers do. But i guess things like these are dismissed as real skills because they are often seen as "feminine" and, therefore, frivolous.
"Serena never lost because she were outperformed, she just lost when she made a mistake"
First off, if she made a mistake on her performance and the others didn't, then she were indeed outperformed.
Out performing means to perform better than someone else. If that someone else commited a mistake and you didn't, then you did outperform that person.
Reminder that she wore this outfit and broadcast herself all over Kalos television:
So, I definitely won't say she's "shy/quiet" or had much of a "shell." She's awkward at times around Ash because she likes him, but she had no problems interacting with everyone around her.
A girl's clothes do not define her personality.
I can be as shy as possible, but still like to wear short skirts and cute dresses.