Decided to give this hack a shot and well... It could have a bunch of flaws fixed, because it honestly needs to be tweaked a bit. The title screen's borders, for instance, are miscolored for some reason. Not to mention that it's weird for the first level to transition from an underwater level to a switch palace out of a sudden. You should be more consistent with what themes you put into your levels. They felt very easy as well (not that it's an issue).
I wonder why the entrance at Grassy Hell is at a blue pipe when you have quite the content some screens before (but you could've expanded it further than what's there).
Be careful when you have a key, because you can skip through the entirety of the level with it by jumping while holding it and beyond. I recommend you make Mario small when you touch a midpoint due to the fact that turning big breaks the entirety of the levels that have it. The following hex edit is ideal to prevent that:
header : lorom
As a final note, while this is not bad for a first hack, you left many things pass before your eyes, rendering your levels breakable. Be familiar with what things are acceptable and what aren't by playing some of the accepted Kaizo hacks both in the section and in the subforum, and the key for an almost flawless level is to use your imagination. Good luck!