I can sum this level up in two images, but I'll give it a full-on review anyways.
I suppose I should start by complimenting you for what you did good with this level - the atmosphere was solid and the coin placement was decent. The level was of moderate length and not too difficult.
AANYWAYS, since that's out of the way, here are my gripes with the level:
The level felt incredibly rushed. It felt like areas just kept repeating and repeating with nothing interesting added to the mix - except that one time I died to the first Fire Plant because of its weird timer. The area in the first screenshot actually exists twice in this level. The same thing! A poorly placed paratroopa below this unfortunate block formation containing coins. Sloped areas also just kept repeating without really changing. Adding a few more enemies into the mix, such as spinies or nipper plants would have helped the level a great deal. Even the ending was missing!
I've barely even touched on the topics of NPCs, and the two points I mentioned above are minor things. The real issue I have with your NPC placement is that there are way too many for the one powerup you're given at the very start.
One thing I was rather disappointed about was that you only had one secret area, also right at the start. It felt like the more the level dragged on, the less love was put into it, which is just really unfortunate. Another secret area would've been nice. Maybe a pipe leading to a bonus room? An invisible 1-Up block at a relatively harmless location? It would just make the level much more interesting if the player was able to find anything in it apart from the endless barrage of poorly placed enemies.
Half of those enemies - especially in the beginning of the level (there are many pits in the beginning of the level, they were scrapped in favour of slopes later on, which is also kinda sad imo) a lot of the NPCs would serve no purpose other than falling into a pit. The red paratroopa also posed no threat because it just bounced off the pipe into the other direction. Speaking of Paratroopas - don't place them in a narrow area. If they bonk their heads it's really easy for the player to get ambushed. I suggest you playtest your levels more and look at your level from a standpoint of someone else playing it: "Will people find it fair if I put this enemy here, or should I put the enemy two blocks further to the right, or leave it out entirely?" Those are the questions you've gotta ask yourself if you want to place your enemies in a reasonable manner. This wasn't reasonable, it was just spam. (Try to convince me that goombas placed 2 blocks in front of a cliff are solid placement.)
There isn't really much more about this level. It's repetitive, spammy and just straight-up unfinished. Sure, it captures SMB3's blockland atmosphere well, but it falls short in just about every other aspect. Also, gotta love the cramped block formations which are everywhere (no, seriously, why do you place the lower row four blocks off the ground, and the second row three blocks above the first? Make up your mind!)
- The atmosphere was captured well
- The level started out well, with a secret to find and it was going somewhere with a large number of pits
- Poor NPC placement
- Poor powerup placement
- Repetitive design
- Gradually decreasing quality