awesome job here - game is fun to play, the song is a lil bop, the movement is smooth and fluid and it shows
overall, i think if you were to do more with this past the demo, i would suggest adding some more personality to the game in the form of bad guys / villains, and a story of some kind. i think the gameplay is very polished and good, and it's worth thinking about & expanding into the questions of, who is this lil penguin guy and what's he up against? you could show this thru cutscenes, or just thru the gameplay itself, but I think putting effort into this would really help bring this up a step from sonic derivative game to charming n' goofy platformer
but yeah, for a newgrounds demo, great job!