This is a nice retro game. Overall level design and difficulty is good, however, the bosses are far too easy.
The first boss is, well, okay as a first boss. The second boss actually get easier each time you hit it, because when he's moving faster, you can jump over it without accurate timing. The final boss, well, you can easily beat him without losing any health.
The problem with the bosses is that there isn't any randomness. (Apart from the second boss which randomly hops out of the water from the left or right pit, but you can just wait until he jumps from the pit you're targeting). You can tell exactly what the boss is going to do based on your actions. The fish will always match the height level you on (on a ladder? on the ground?). The final boss will always target you with his fireballs. Seriously, you can just run back and forth, making the battle downright boring.
I think the boss battles need at least some elements of randomness. Let the final boss spawn some more fireballs that are just falling from the ceiling at random positions while he shoots for you, or something like that.
Also, it gets boring collecting 999 Kupons, I'll skip that medal.