The overworld looks cutoffy and the level could be improved with ledges or something.
Those black stars surrounding the title screen look weird. Either replace them or remove them.
Stacked munchers. Put a ledge under them or something.
Grammar error. The text could be replaced with this:
"Wow, the Bullet Bill didn't kill you? You have such an ability! Welcome to the Island of Tiles, and good luck!"
The trap is OK, but you could remove the two first note blocks.
(In case you wondered, I killed the Goomba, which isn't needed at all) You could remove that Goomba.
You need to enable vertical scroll there.
Where's the midway tape? And I carried the shell from before throughout the level, I recommend making it impossible to get, by putting cement blocks or something on both sides of the Koopa that I got the shell from.
Again, stacked munchers. And also, you could remove the brown block I'm standing on because I can skip it.
Is that coin needed?
Floating munchers, and also cutoff.
So yeah, you should fix those issues I mentioned, and also, you should read this
if you want to make a better overworld.