It seems you are using NPCs not present in tileset itemboxes by default (smb1 fire spewing piranha plant, maybe more?). NPCs that cannot be found in tileset itemboxes, as described in the handbook, are unfinished and subject to change in major ways.
Added in 11 minutes 43 seconds:
Nice rompy desert level overall. Large difficulty spike between dolphin segments and the rest. All non-dolphin enemies are trivial to skip over and often placed in a flat-surface environment. Sumo Bros suffer most from this, posing no threat at all throughout the entire level. I died an insane amount of times in this level, but always while dolphin bouncing. 2nd half much harder than 1st half, but powerups would scarcely help because you die in one hit.
Small personal nitpick: Level uses bottomless pits as well as oil instakill. Why not stretch the oil as a foreground instakill element throughout the entire level?
After jumping this gap I realized what the shells were for. Oops.
E: sorry for the super brief and hacky response but I was under strict time constraints. Typing from the train to let you know that more specific suggestions are coming later.
E2: As promised, some more specific suggestions.
I've recently started noticing a tendency people have to stretch setups out for a few blocks longer than they ever need to be. I've started combatting this urge in my own design only recently and noticed quite a few instances of it here. You'll notice my suggestions are often shorter than the original.
Feel free to take as little or as much as you want from these. The areas I highlight can be changed in a multitude of ways (or can be kept as well) and I merely highlight the one that came to mind for me.
