So, after playing it few times, here's my review:
So probably what you've said in the OP, the level is bad. The first sections have some decent design, but the other ones lack BGOs and flat design, which makes the gameplay boring. The boom boom boss battle was nothing interesting. The red tree leaves need some work, since they look weird. NPC placement is kinda okay, but sometimes you add some unnecessary emenies and obstacles, especially in one screen, you
add coins under the ground and a goomba in mid-air. Also,
in this part, I'd suggest you add some directional coins in the sky because that will make to find the pipe much easier.
So, after many good things and bad issues, this level deserves an average score.
4/10