[center]So, I sat down, pulled out SMBX, got out my reviewing music, and reviewed your level:[/center][/i][/b]
So, when we start off, we have a pit of blue lava. I found it quite obvious it was lava, but some other people might mix it up for water. Maybe add a sign or infomation speaker to alert the player to the danger of the lava?
Continuing, we have quite a... strange random enemy entity? I'm not sure what happened, but he didn't move and literally just sat there not moving, strangely flashing for me.

Jumping on him results in a spin jump bounce, regardless of if you spin jumped. Running into him from the sides will kill you however. I just found this very strange, and both me and Mario are quite confused.
Moving on, the level is extremely short. And when I say short, I mean short. I beat it casually in 45 seconds which is not very good at all. The level is quite bland, featuring mostly empty rooms void of any decoration and enemies. The platforming wasn't too bad, but honestly it was extremely easy. Enemy placement was very questionable at times, and the custom blue Koopa Troopa graphic you used makes it impossible to distinguish if they are red or green, but it wasn't too big of an issue because there weren't any scenarios that affected Mario in this way. Back on enemy placement, there was one Koopa Troopa I am going to use as an example of strange enemy placement.

The poor Koopa Troopa you see above starts around the Blue Door, but ends up stuck in this pit where he is virtually pointless. Making him red would make him more of a threat, but at this point he isn't and Mario can just safely wait for him to walk into his doom. Anyways, the cyan bricks you used, I'm not a fan of them. I think it's because they have no outline. Look, it's just decorative OCD but it really annoys me. Coin placement is very basic, but I felt like there was too much coins.
The boss was quite easy, but as our pal Taycamgame said, there is no "refreshment station" before the boss. A "Refreshment station" is a powerup and possible checkpoint alcove right before a boss, to, well, refresh the player and give them a break from the action. It's important you add this for your players so they don't find themselves in a tricky situation where they're a tiny little plumber up against a massive brute of a boss. But fortunately the boss was easy so for me, I didn't need a refreshment station.
The level its self wasn't too bad, I wasn't enjoying myself but I wasn't having a horrible time. Maybe some kind of ice physics related gimmick would have spiced things up?
Anyways, your level was below average but don't let that discourage you. You can only get better the more you create! Thank you for spending time on this level and putting hard work in it. Hopefully you'll take my advice and use that in the future.
[center]Overall Score: 4.5/10[/center][/i][/b]