This was pretty nice. I did stumble upon some strange graphical screw-ups when I was holding a shell while looking up, and while moving down in water while small. (1)(2)
Also, you may notice my lives count is really high in the first screenshot. I found a tiny space for a moving shell to get stuck in right in front of one of the bullet bills. Convenient, but also rather broken.
In the ending castle level named Iggy's Castle there are some sprite platforms that you need to use to jump to the higher levels in the side to side movement layer 2 area. Due to the bowser statue shooting fireballs that gray platform sometimes doesn't spawn. That means that you can't beat the level without getting hurt. So the level is kind of broken when those platforms don't spawn, you may want to change that.
Quite short, but it was alright. My main suggestion for if you plan any future updates or sequels would be to create some sort of visual representation that tells the player what powerup state Monty is in, since right now they're all the same. I do appreciate the hitbox staying consistent though!