I thought this was a cool little platformer. I also appreciate the mechanic of "stuff changes when you jump", it was used to good effect in certain areas like making you go against your instincts and jump right towards thorns that you know are going to disappear.
I felt that the hitboxes on the thorns were a little unforgiving and I would have liked to see some checkpoints.
In terms of "era-feel" you say you were targeting Gameboy screen resolution, but it doesn't really read as a gameboy game visually in any other sense, and there are mixed pixel sizes around that definitely ruin any illusion that this was made for retro hardware.
Overall it's a good game, especially given that you say its the first game jam you've done and the first game you've made period. Excellent effort!