The reward for using less tiles idea would have made this game so much more replayable but yeah in game jams you can only do so much before time runs out.
In regard to difficulty I don't think game jam games have to be easy , they can obviously, but in my opinion it shouldn't be a rule to make all games easy. It's important however to ease the player into the experience with an slow start and then slightly increasing the difficulty to the point where you want it to be. The problem in my game however was that the mechanics aren't clear enough for some people, even if the platforming is easy, the mechanic needs to be understood. The reason my game isn't that well explained is because of speedrunning reasons btw.