I just finished this game, it was pretty good! The platforming was decent, the graphics looked good, the music choice was good.
The boss was decent as well, but I felt that the first phase was a bit too hard. It's definitely harder than the 2nd phase, which seems a bit backwards. It was funny though, and some attacks were creative.
Overall, this is a very good start for your first fangame, I can see tons of potential. Looking forward to your future games.