Since you've been angling for some feedback, I decided to have a go and after playing the first world (I got stuck on the first castle, I'm no good at SMW hacks

) and it seems pretty good so far!
It manages to "feel" like a Mario games fairly well; the aesthetics and music choice fit in well without feeling too bland.
Level design is fairly good, although the difficulty seem to be bit inconsistent (although this might just be me being be at SMW hacks).
A few complaints & nitpicks:

This fall happens pretty fast and doesn't really give the player much chance to figure out what's going on before the die. Add a few coins to show that a jump is needed.

The level clear screen is pretty buggy: the bonus star tiles look weird and the "COURSE CLEAR" text and such only show for a frame (1/60th of a second) before vanishing.

Small issue: when you kill a chuck with fireball, it flies
way over the right, which is honestly pretty funny.

For whatever reason, these fuzzies "crawl" along nothing, which looks odd, I'm guessing they're supposed to go around the outlied blocks and you didn't orient them correctly?
also can't make this jump for the life of me

Like the jump I pointed out earlier, this comes to fast for you to react easily and I think some coins should be added as indicators.
NOT PICTURED: Some lava in the castle isn't extended all the way down, causing cutoff when you die.
In short, it's good so far, although I feel like I need to get past world 1 to make a fair judgement.