Nive level ! It's a good start! This is my personnal review :
gameplay: 7.5/10 : It's a classic level, as we can find in world-1 of mario series. The 3 traditionnal sections (sky, ground, underground) are here. I regret to get too easily some rare power-ups, like 3up moon.
Design 5/10 : Well design, but i see some things that was ... weird. As SMB1 blocks or unfinished pipes. You also put SML2 flying goombas with your custom goombas. And, why did you use an underground with NSMW graphics and you put smb1 and smb3 graphics for the overworld and the sky section ?

-> SMB3 and SMB1 block together in an overworld with a NSMBW backgroud and YI koopas, it's strange

-> Why SMB1 blocks here ?

-> SMB1 blocks with a SMB1 pipe. In all the level you uses the SMB3 pipes, so why here ?

SMB1 block with a pipe with mixed graphics (SMB1 + SMB3)
Music 7/10 : The music was fine but i don't understand why you put a super mario galaxy song in a sky place.
NPC Placement : 8/10 The NPCs was nice. But some were ... strangely placed :

Oh well, i'm a flotaing plant !
My final note : 7/10, good for a first level
[What's the ground section theme ?][If i made grammars errors, say it to me, i'll fix it]