I don't like the 4th level. It feels off from the rest in how dangerous and how precise the platforming is. Further, it feels like it's designed to make the player have to backtrack, which is irritating. The 5th level feels that way a bit too, but it's not a problem to backtrack for a bonus item, in my opinion. However, the 5th level also feels out of place in a different way: it feels like it would've needed more levels leading up to it. The solution for dealing with the pit of spikes requires more comfort with the mechanics than I would expect the player to have at that point. If you do choice to continue working on this, I would recommend both of those levels being moved to later in the game. I would also recommend introducing spikes in a smaller way.
Those two levels sit in contrast to the first 3, I think. The initial tutorial is excellent, though I may have missed any mention of which button is jump. The bigger thing missing, I think, is a way of learning and getting a feel for how the rate the time power goes down is determined. I had the impression at first that it was always the same, then only later noticed that it seemed to be different sometimes.
Overall, I hope you do finish the game at some point. I think pixel art games are never a bad thing, and I think there's a lacking of platformers with cute bunnies and interesting takes on the mechanics.