A few things that make this look sloppy to me:
* Coin blocks that aren't possible to hit, used as floor
* Leftovers from the original levels. It just makes it obvious that you edited them instead of starting from scratch.
* Awkward background placement: half a bush, middle part of a bush, etc.
* Floating bridge that isn't connected to anything on the ends, not even floating blocks.
* Over-use of difficult jumps from a single block to a single block to a single block. This isn't a complaint on difficulty, it's just boring. Don't make every jump a difficult one without enemies around, make them medium difficulty then add elements to make them harder -- lakitu, flying fish, moving platforms, springboards.
* Misplaced enemies: Bowser floating in the air, and blocked by invisible wall, koopa stuck in blocks.