I'm a little sad that you weren't able to use those smog bubbles you had originally intended .. oh well. At least you introduced Yoshi in a rather nice way, which is great.
You might be better off using a red Koopa here instead of a yellow one. He walks off the cliff and gets stuck here, thus looking really weird.
Might I suggest opening up YY-CHR and giving the top of these fences a black outline?
Are the Munchers surrounding the block snake trail really necessary?
The part below is honestly rather blind. When I first jumped down there, I expected to land on something solid, but .. well, since you need the wings, you just fall to your death.
It might be cool to put a 1-UP/mushroom/some coins here (unless I missed it on my playthrough).
Other than those bugs, it's a nice little level. Good work.