Okay, to get rid of confusion, let me get this straight. When you complete a level with all chests opened and all 100 coins collected, you get a star on the level in the menu. When you complete a level on a certain difficulty, it will show a certain letter atop the star (if you got it). E = easy, N = Normal, H = Hard, S = Epic.
Let me tell you a story. Once upon a time, I 5-star'd and faved a game. This one looks vaguely familiar to the one I loved and cared for. You outdid yourself kupo707, keep making these great games!
Easter Egg(?) Found! Go to level 11-2, and on the top of the level, in between the furthest two pillars on the right, you'll see a Kirby statue made of snow! :D
 "Cell Warfare"){kind=icon}
 "Pixel Purge"){kind=icon}
 "ESCAPE"){kind=icon}
 "Bullet Heaven"){kind=icon}
 "Midas"){kind=icon}
 "Epic Battle Fantasy 3"){kind=icon}
 "Factory Balls"){kind=icon}
 "Adventure Story"){kind=icon}
 "Happy Wheels"){kind=icon}
 "Super Crazy Guitar Maniac Deluxe 2"){kind=icon}
 "Bosses!!!"){kind=icon}
 "Paladog"){kind=icon}
 "Burrito Bison Revenge"){kind=icon}