The left is closer to show style, yes, but the artist’s later style on the right is a great measurement of their improvement. The wing is better, the forelegs don’t look like macaroni, the hooves are defined, and Fluttershy looks fuller and less two-dimensional.
 
She does appear to have a little pudge now, but the visible bone joints shows a good understanding of anatomy and where good pudges would logically be. If the Shy on the right didn’t have the joint visible on her ankle, for example, it would suggest a build up of fat tissue over it - which typically occurs when there is too much fat tissue all over the body.
 
I just went off on tangent, but I’m going to let it stick. What I wanted to say at the very beginning is that I like how the artist has taken to developing their own style and has a different interpretation of Fluttershy’s design.

I liked the one to the left better.:/

I like first one more.

Is this supposed to show artistic improvement or normal Fluttershy versus chubbyshy?

Both are still goddamn cute.

Good improvement