Good
You said that you hadn't read it, and given that criteria, or even w/o it, it's a pretty faithful adaptation.
@RYUHAYABUSA5009 In the story, the secret ingredients for her cupcakes were pony innards, and the story (along with this film) depicts her harvesting that ingredient. Thus, Cupcakes.
@CyperAleksi you aren't the only one. I believe that, since she seems to have little concept of the long-term implications of her actions, we view her as sick and insane, and speaking for myself, I actually felt sorry for her. In the original story, I thought the violence sometimes went over the top (as in humorously so.)
Animation was OK. Scenes where they are talking could be better.
Music: No comments. Nice choice.
storytelling style worked well here. I think Pinkie's expression is a bit overstated, but that's just me (I would have gone for a more "Party of One" look, though it seems like that was what you were going for, with the straight mane.)
Overall this is pretty much how I envisioned it when reading. I like the details, with the writing on the wall (no pun intended.) It's good.