I am unfortunately not a huge fan of the ground/platforms and the characters. But the game seems to work really well using squares and simple shapes so I really think you should lean into that. I would recommend looking at other games that also use simple shapes to create appealing visuals.
Here are two games for this jam that I think does that really well:
Switch https://itch.io/jam/gdevelop-game-jam-4/rate/2216920
Switch Puzzle https://itch.io/jam/gdevelop-game-jam-4/rate/2209769
I also made a game using only squares and simple shapes if you want to take inspiration from that https://atomicpotatoemil.itch.io/shapely-shootout Though I think the before mentioned games did it better.
I can also recommend looking at a commercial game called Thomas Was Alone that made awesome visuals using only squares.
I hope some of this helps, I am very impressed by your first jam game, and I can easily see the quality of your games increase exponentially in a very short time frame. You already got a great handle on mechanics, now it is just about practicing to create appealing visuals :)