Screen 1: Good. At least you centred your text in this one.
Screen 2: The mountain's look very fake, and the BG palette doesn't match the clouds. I think you've edited this..?
Screen 3: Good (Nice idea with the Koopa Shell.)
Screen 4: Another Smart idea.
Screen 5: Cool
Screen 6: Good idea with the wall climbing, but if you would post your hack, are you sure people would know how to do that? Maybe you should consider placing a Message Box to hint about what to do.
Screen 7: Good.
Screen 8: Good, but i'm not sure about that glitched area.
Screen 9: Maybe make your text a little more long, say something like:
Into the Mole Holes, to make it sound better than something plain like
Screen 10: I solved it, where's my cookie?
Screen 11: The stone blocks aren't exactly evened out, one that's 3 tiles long and others that are 2. Maybe consider something else, not stone blocks, because it looks rather dodgy.
Screen 12: Good.
Screen 13: Good.
Screen 14: Yes, infact i know this. Firstly, to make the ceiling, type in "Ceiling with slanted edges..." and put that BEHIND the cross section, just under the snow (or orange block, whatever you used.) For the edges, make the spikes thinner, or the snow wider, and insert the left and right corners of normal ground. You might noticed that the ceiling is then cut off by the corners, just use orange blocks to cover that up.
Screen 15: I might know how to do this...
Screen 16: Good, but is there a point to the spikes next to the pipe? Maybe add some coins there?
Screen 17: Good, but a little crowded. Stretch it out a bit. Can you see how the rotating platform is going through the stone blocks and enemies? That's pretty unrealistic.
Screen 18 & 19: That's a good trick, very smart.
Otherwise, good hack, just fix those things.