I understand that she can only see the bubbles if she's looking in their direction, but since she only faces directly left or right, getting her to move straight up and down is a nightmare. Also, it's irritating that I can't seem to get her to face the opposite direction when I want, since her field of vision cannot turn gradually. Both these problems would be fixed if you would make more sprites for Derpy and let her face north and south, in addition to east and west.
Also, you must, must, MUST change the punishment for failing a level to something less extreme. Instead of being taken all the way back to the beginning of the game, maybe send her back to the start of the previous level.