I feel like the mushroom on top of the mushroom house should be a little lower on the cap, and that the bottom-most part of it should be slightly rounder, because as it is it makes the house looks like a 2D cut-out. The same sort of issue is affecting the Hippo; try moving the left eye down one or two pixels, bring the muzzle to the right by a few, move the right nostril up a pixel or two, and round off the bottom. Also maybe give the Space Zone's star two eyes. Besides that, it's very nice.