I played your level with the original SNES controller, and it was pretty fun design-wise. There were only a few issues or disagreements here and there but it was pretty good overall. Now for some screenshots!
This little oddity only happens with the flying turn blocks that could push Mario through the ground, raising it a tile or 2 should fix this.
I mainly took this screenshot to talk about the level's graphical set, as they tripped me up at a few points. In actual gameplay, it was the bullet bill shooters and the floaty rings that surprised me with solid interaction. I can't really explain why but they look passable to me, my suggestion to fix this would be to make the rings act like tile 100 and to make the body of the B.B. Shooter to be a bit more thicker. Aesthetically, I enjoyed the main graphics (the boardwark and fences) but the decorations cold be polished more. Firstly, you need to lighten the shading of the balloons as it stand out too much to look passable, and could you remove the black outline around the bright spot of the third balloon as it doesn't fit. Another thing that could use some lighter shading is the big tops in the background (and the floaty ring), the white and red should have a smooth transition (Example
of what I mean).
For the enemy graphics, I'm not fond of the pie and Ice Cream Cone + Droplet graphics. The pie has the shape of a pie, but it doesn't really look like a pie (this could be due to palette and I'm sure there won't be an easy fix for that). For the Cone, maybe make the texture larger because I was trying to figure out what it was the first time I saw it. For the droplets I'd suggest using sharp broken pieces of the cone or sharp looking condiments.
Final Oddity posed to sprite designers instead of the level designer:
This guy after dropping a sprite floats to space then comes back down and goes again in an odd bobbing motion. Is it supposed to do that?
Highlights of the level: I had to use my brain to figure a standard platforming level out without frustration. (I was stuck at the first screen with the rotating platform and the invisible blocks, which I thought was switch palace required blocks, I still was able to make the jump without the blocks though.)
Qualms (curse you Hey Arnold for teaching me this word): Sprites tend to disappear more often than usual, disappearing like not appear instead of their graphics going blank.
@ AxemJinx: You need to spin-jump off of the shark torpedo-guys, which is possible to get as small Mario.
I wonder what a HFD opcode would do in ASM...