This is a cute little level, and it was fun to go through. Design-wise, it's fine enough, but I think this level could use a bit of touching up to reach its maximum potential in the visuals department. After all, enemy/land/powerup placement is the base of level design, but visual communication is crucial as well to make sure the player understands the level well.
First, the foreground palette could use some desaturation. Some of the palettes, like the pipes and vines, are great and fit the atmosphere well! However, the ground and lava are maybe a bit too saturated. I'd suggest reducing the saturation a bit, and in the lava's case, making the palette a gradient (which Lunar Magic's Palette Editor lets you do if you define a start and end colour, and press Alt). Currently, the greenish top part of the lava feels sort of disruptive. Something like this
might work better. Of course, this is just a starting idea, and I believe you could go even further with it. This is applicable to more than just the ground, so make sure to play around with all the palettes.
Moving on, some of the sprite palettes could use touching up to be smoother as well. Like with the foreground, some things, like the Mushrooms and normal Boos, look really good and blend in well with the level's atmosphere (although perhaps could I suggest giving them a slightly different hue, like purple, to further distinguish them from the foreground?). Other stuff, like the Boo Venus Fire Traps, leave a bit to be desired. The key here is to try and make smooth gradients so there's a consistent colour scheme. Here
's a preliminary idea of something you could do. Once again, experiment to your heart's content.
Finally, you could use colours to demarcate level elements more skillfully. I mentioned it before, but slightly changing hues between the foreground and enemies can do wonders to give the player a better grasp on what's what on the level. Another important example is the flames; as hazards, they should stand out from the rest of the level, as they otherwise risk blending into the background. Here
's one example, but again, you can go further.
I know I've written a lot, but don't take it to heart, I think this level's got some potential, and I want to see it in its best shape. I also encourage you to experiment beyond the examples I've given, if you want to use them, as you know best what you want your level to look and feel like.
I hope you find this helpful. Best of luck with everything!