Hello there! So let me start with the positive points first. The animation was pretty cool looking! I don't know why but I felt like an NBA player shooting some hoops! Also, I really like the wall grabbing mechanic. It was very well implemented!
Okay, now for the stuff that I think needs improvement..
I found the game too dark. When the shadows were gone, the game felt way better. If you want to add shadows, try not to make it too dark and also I suggest adding a faint glow around the character, as if it is also a light source like the flames.
When time slows down, the player jump height decreases. Not sure if that's intended or not. I would prefer if you could still remained the same.
Another thing, I got stuck in the level where only the mouse can enter the small hole. There was a sleeping monster but the mouse just kept on going back and forth, the monster didn't wake up.
I think it would be nice to incorporate cayote time to your platfomer. Cayote time is allowing the player to still jump after going off the ledge for a short period of time. The reason why you do this is that sometimes, the player presses the jump button too late and they are already in mid air. However, the player believes they pressed the button when they were still on the platform.
I apologize if my review sounds too negative. But honestly, I was checking the GMTK comments and all they were saying was "well done!" "congratulations", then when you check the ratings, you could tell most rated low. I'm wondering why they even bothered praising you.
Anyways, I got too carried away. I hope this helps you! If you need some help, I am also a GMS game developer :) I have experience in making platformers with GMS.