For the ones who don't really want to read the text below there is a short summary: Serena's rivals are useless and not the rivals at all.
Not only they're the worst rivals of pokegirls existing ever (even Misty's CoTDs in OI and Johto had more personality), but they make the entire Serena's progress towards her goal kinda pathetic in comparison to both May's and Dawn's rising towards their respective Grand Festivals.
1. Miette
She's a fun character with solid design introduced early, but she's just the tool to fuel AmourShipping and making Serena nervous about revealing her crush.
She always was inferior to Serena in Performances, she never really give her some challenge even through some psychological manipulation and gaslighting, not even skills.
2. Shauna
She at least informed Serena about Showcases and the rules of competition, then became irrelevant.
Again, she never won any showcase on-screen bar Coumarine city one. One solid victory seems okay, but, tbh, it means nothing. Shauna always lost to Serena in all the next showcases, so the reason of her earning the Princess Key isn't her competence, but Fennekin's long bows.
With more experience and the most friendly attitude to Serena she could became Zoey-like mentor character teaching her apprentice and watching her grow after some close matches, but that opportunity was pretty much wasted.
3. Nini
Don't even know what to say about her honestly. If the writers wanted to show us that not anyone fits Showcases (and performers' popularity is based on their looks too), she could be a nice character of the day.
But girl reappeared many times and was very bad in the field of EVERY showcase section. She can't make her pokemon move gracefully (Farfetchd's case), she can't style herself properly (horrendous gothic lolita outfit with violet lipstick), she's dumb (failed Pokemon Quiz).
How could the character that bad could even be a rival? Cameron at least had some powerhouses.
4. Amelia
Opposite to Nini, she appeared only in one episode, but would make a great reccuring character. With cynical and posh demeanor, solid fanbase from her hometown, great pokemon like shiny dragonair and espeon her chances of following the footsteps of both Gary, Harley and Ursula and becoming the powerful rival could be pretty much high.
So all the rivals (I'm not counting Jessie) were kinda... here? They brought no challenge to Serena, they never defeated her, they never taught her something new and had zero influence to her goal.
After Dawn's and May's rivals they're all lackluster.
It's Serena who's always a better performer, it's Serena who's gave Nini advice about Farfetchd etc. Nothing wrong with the latter, but dynamics and sharing of experience could be more... Mutual? Reciprocating?
I know that this sub is filled with army of Serena's fans downvoting, but she could be an outstanding character if we've got less amourshipping pandering and more actual struggle.
Not only her rivals, but the whole Showcases plot suck.
Serena never lost to someone more skillful and proficient than her bar Aria. She lost not because of other competitors, but herself twice (with Fennekin tripping and Eevee being shy). Someone could say that it's a great way of showing that Serena's biggest obstacle is she herself, but truly attentive performer at least had some practice and talk-no-jutsu with Eevee.
Even the first section of various Showcases gave Serena everything on a silver platter. She's already good at styling and baking, and out of sudden appearance of rhyhorn's horde for the daughter of famous rhyhorn racer?). Instead of pushing her to the limits and honing her skills the writers chose the most lazy way for her always be the victor.
And tbh being the naturally talented, superior performer with the competitions almost always coincidentally matching your hobbies isn't a good writing, but the bad one.
Yeah, Performances are more like beauty pageants competition mixed with dancing, figure skating and gymnastics as inspiration, but they still could work better (and take the real dark sides of teenage girls competing).
Imagine Shauna being more experienced and skillful performer who's always one step ahead, so Serena should work hard and constantly train to exhaustion.
Imagine Miette being less proficient, but anonymously blackmailing Serena each time the showcase occurs, threatening her to tell Ash about her feeling or even reveal it publically.
Imagine Nini being unattractive, but very smart girl with unusual techniques, moves and combos; Conway-like style.
Imagine Amelia using her money and influence to bribe voting viewers, boost her fame with massive amount of advertisement or buy some rare pokemon or items (maybe the incident with rogue mega evolution due to the lack of bonds?).
That would be a totally different path towards the title of Kalos Queen and logical continuation of writing quality for coordinator/performer girls.
You could start arguing that Serena had less episodes then both May and Dawn, but three princess keys are less than five contest ribbons.