Loved learning the mechanic and starting to get into the groove. I think the difficulty is basically how far away something is from where you jump - so for example, the higher the enemy is, the harder it is to hit. Maybe another UI layer that helps give you a little heads up of where you might appear at the top of the screen would help. And also maybe slow the fall slightly right as you're switching to the top of the screen to give you some time to re-correct. Also some leeway on where you land on the head vs when you get killed. 

Once I got into the swing of things I was able to fly through the game, though, so I'm just throwing out random ideas you can completely ignore!

Very nice mechanic, my only issue with the game is the hit registration for landing on a head is a bit harsh if you are a tiny bit off. Other than that, very neat platformer! Would like to see more levels for this :D

Like a dream...

Nice concept, I always want to jump so it frustrating :)