What a cute game, and really great for your first game with Stencyl! :)
I really loved the idea of lighting up all the colors of the rainbow to unlock the gate. It was a really colorful and visual way of displaying a player's progress. I also thought the creative game over/retry scene was really neat!
I think the game is great with a wider platform field (I played a little earlier and had problems falling out of the clouds!) but I can definitely see why it might not visually be an ideal solution. For future games, you could consider using primarily block shapes (like clouds in the shape of a block) or Stencyl's custom collision shapes, for more precision collisions perhaps?
But overall, love the effort you put in! :)