I think I would enjoy it more if some of the personality attributes of these two were mixed.
If fluttershy was still shy but very adept at flying and sports and confident in her own abilities, at least. The other ponies might originally have a habit of overlooking her and the things she can do because she’s quiet/she’s too shy around them to ask them to come see her compete in some of the flying games, etc but it doesn’t mean that she can’t do them just because she’s shy about asking.
I would just really have enjoyed seeing the token shy character be really good at something and enjoy it.
And rainbow dash the braggart could have fluttershy’s anxieties. She’s great at flying but only actually does it when she can’t think about it ahead of time, but it doesn’t stop her from bragging about how awesome she is because she really does know that she’s awesome. So what if she’s a little bit anxious about flying alright. And all of her competing with Applejack came about because of how much time she spends on the ground.
I dunno, i think it would be cuter on both of their parts. Mostly fluttershy, I wish there was more to fluttershy, haha.