Hi!
For a first Demo of your first platformer, this is a very promising build!
Controls are solid, I experienced no bugs or errors and never once did I feel like I wasn't entirely in control of the main character.
Graphics are charming and consistent and the level design is more than varied enough for a short demo such as this one.
Music is neat and perfectly fits the 8bit vibes you're aiming for.
If I could suggest anything to add or work on for a new version if you work on it; possibly a brief set of instructions for player controls. Maybe a timer also, so the player has even an easy challenge to face when playing.
Best of luck with your future development!