This is a bit of a long comment but I feel like the feedback could hopefully be useful. I will start with saying I liked the art. the music was a bit too loud and a audio slider would have been nice (but even just lowering the volume by in general would have been nice (not that big of a deal). The countdown seems very loose but also sort of detrimental to the game itself, like if it changes mid jump near spikes you just die but also for one of the sections after a checkpoint each time you die you just have to wait a prolonged period of time to turn into the negative mode just so you can be small which makes dying just feel too punishing. I also got tripped up a bit with the moving platforms due to a combination of 2(ish) reasons.
1. the player not keeping the velocity of the platform.
2. a mix of the fact the platform moves back when you aren't on it and the background making it so you don't really immediately tell it is moving backwards.
I was at the front of the platform at the first part and was very confused how to go further as the game just made me think the platform just goes backwards from there.
hopefully this feedback is of some use and sorry if it comes across as overly negative, that is not my intention.