Not bad. This level does just about everything a 1-1 should do, though the mid-boss felt a bit weird. Not bad, but weird. There are a couple design issues though, one is in the first section, it is possible to jump up above the top of the level through the waterfalls and get stuck. The other is in the 2nd section where you can glitch through the wall by facing right while standing on the conveyor belt (which felt out of place) and jumping.
Npc placement was good in my opinion. I never felt overwhelmed (its 1-1, so it shouldnt be kaizo just yet) the death animation (fireball/tailspin) for the goomba was different than the standard SMB3 goomba you had though.
All in all, a solid 1-1 in my opinion. Well done
EDIT: I would also like to point out that it is possible avoid backtracking for the springboard if you fly using the leaf provided. Im not sure if that was intentional, as the ground is uneven and flight is difficult, but it is a nice option for those who hate backtracking, and you have to get to that point without getting hit to be able to do it, which is a nice tradeoff in my opinion, especially since this is only 1-1.
My (now finished) Project: The Legend of Peach
Does anyone still make project banners? Mine isn't a large project or anything, but some of them look pretty snazzy. PM me if you make them, and wouldn't mind making one for me.