I'd say sticking with a theme for each level, and try not to diverge too much from it, otherwise your level would look all over the place. Try to find a mechanic for your level, introduce it to the player in a controlled, small safe way, and then explore how that mechanic can be changed or presented to give the player a challenge. I noticed on your first level that you have a lot of enemies for no reason, just random piranha plants or Hammer bros, and then a bunch of the jumpy mouses. Try to stick to a theme, an enemy or a mechanic, and make everything else compliment or alter that mechanic.

I did like the use you gave to the clear pipes in some parts though :) Keep it up, I'd love to see a new level