Awesome level you got there. The boss is amazing, the music is good and the overall level design is ok.
A few (small) things:
When you beat King Cranky, he still uses the mother brain dying effect. I believe this is effect 112, and a simple change to the first frame King Cranky should be fine.
Spoilers for the ending:
In the first screenshot, the cheep cheeps on the right are lower than in the second screenshot. They randomly move up 1 tile after the cutscene.
There are also 35 green cheep cheep in the pond at the start and 34 green cheep cheep at the end of the level
but who counts the fish anyways
I really liked this level, keep it up.