"Derpy will no walk into any harm on her own"
If only this rule applied better when Derpy gets hit by anything (most notably happened to me in the stage with the log you had to time to walk over it), because despite some of the invulnerabilities (as I presume there is some time window for that), Derpy becomes a tad too hard to control, and I managed to lose three lives for one mistake because I couldn't her to behave properly before she bumped into something again.
Also I'd criticize the choice to move camera in accordance to the mouse's position. Because of this it takes a couple of seconds to locate Derpy when a stage starts, and it gets slightly irritating in any tighter area where you need to be careful. I'd suggest to centralize the camera on Derpy when a new stage starts (if it's already supposed to, then it's not working right), and make the camera move only at the edges of the screen. Perhaps make it an option to decide between the two camera types?
Biggest gripe for me however is the decision to throw the player all the way to the beginning upon game over. In a simple nature of a game like this, it only becomes tiresome to go through the levels again, even more so if you just did one simple mistake that then lead to three-four hit combo and game over, like what happened to me a couple of times. Players may outright just quit and not finish it because of this. it's only superficially padding extra length for the game.
Other than those, it seems to be a fairly good game with some nice looking in-game environment graphics to boot. Music fits quite nicely, and Derpy feels pretty lively. The cutscene animations aren't too bad either.
All in all, I'd say it's a fairly simple artsy game, with some rather minor, albeit annoying, flaws. Solid 3/5 for presentation alone.