I honestly really relate to this moral a lot. I can understand that you're supposed to get better with practice, because practice is supposed to be good, improve your skills and all that, but for some people having pressure put on you for something can be utterly miserable, and suck the fun right out of something.
It's something I actually have experience with. I used to really like writing when I was a kid, I enjoyed writing stories of all kinds. They varied in quality, some were kinda bad, but I started to write some stuff that was overall pretty decent, I was kind of proud of it, even had one teacher call my mom in for a meeting on how good one particular piece was.
Then the pressure came. My mom wanted me to write stories for her, everyone was pushing on me to develop this talent, write better and better, practice, practice, practice. It started to get frustrating, I wasn't really enjoying it anymore. Sure, the practice was good, but the pressure, and the constant pushing to do this thing even when I didn't feel like it just got unpleasant. I would procrastinate writing because I just didn't want to do it anymore, and the quality of it started to go downhill fast.
I probably never would have been a "Professional" writer of any kind, I really doubt that. But it could have been a fun hobby, I could have written fanfic's or something, or if I really stretched I maybe could have gotten a bit of money from writing requests. Except I haven't written anything in years, and I have no desire to. It's just not fun anymore, I think of it as work, it's nothing but a chore, and nothing's ever good enough.
So, I really relate to Pinkie and Fluttershy here. I don't think they wanted to play the game "Professionally", they just wanted to have some fun, I don't think it's that they didn't want to practice at all, but the harsh regime, and all the pushing by everypony to do well got to them. For Rainbow and AJ who are really into athletics, they thrive off this, but for ponies like Pinkie and Flutters, it's detrimental.
Also, Snails was great. I never thought I'd say that, but he was. I'd like to see more of him like this.