Stardust Balance

As I understand it, the 2017 film wasn't even that loved. I found it just plain average.

If she was a mary sue, she would've been loved by everyone off the bat. Sunny does get challenged numerous time; by the townsfolk, the other races, her own doubts after the first attempt with the crystals failed. And many people as adults have the same personality as they were a child. It took a bit for Hipp, Pipp and even Hitch to learn to trust and join her; Izzy is just a loving character at heart. The royal pegasi family are essentially a group of divas, and it's unlikely they get many prisoners regardless. It makes sense narrative-wise why the prison would be more luxurious than stone-cold cliche dungeon. How is that not development? Her flaws were being presumpteous and completely certain of her thoughts without thinking clearly. Sounds like a certain lavender alicorn. Sunny learns it's not the crystals that what unites friendships, but the individuals. That sounds like development to me. I repeat my above statement: Sunny does have flaws in that she's overconfident in her beliefs and completely certain everything will go her way without any second-guessing to the holes in her ideas and how she goes about them. The defence presentation, the crystals, the dance-off. She gets shown otherwise numerous times and it helps her learn everything is not as clear-cut as she expects it to be.

No but doesn't he live on top of a mountain, just as the pegasi do? I think that's the reference.

Not really a surprise; she had the least amount of development and screentime among the crew. I'm sure the series will fix that.