Thank you for your kind words and feedback!
Regarding the scoring system. At first I wanted to have a classic life system instead of a score (like you start with 3 lives, if you get hit you lose one, and game over if you lose all your lives). But since it's a game jam, I wanted people who play my game to be able to see all the levels without dying.
So I removed the life system and implemented this scoring system. The scoring system works like this: Your score increases when you kill an enemy, a boss or when you complete a level. If you get hit, you lose 10 points. The maximum score should be around 180-185.
You are right, I did not think about it, but I should add a +/- score pop up, as you say, close to the character or maybe on the top right, close to the score?
I'm thinking of updating my game after the jam and if I do I will definitely implement that.