There's a difficulty spike in the fifth level (green switch) and an even bigger difficulty spike in the sixth level (baby Yoshi). The sixth level is insanely tight with some tough, input intensive maneuvers. I lost the same amount of lives in that level (300) as the rest of the hack combined, and I would say it should be changed if you're promoting your hack as easy/intermediate (the rest of the levels meet that description).
Overall, I wish the level design had a bit more originality and creativity. I don't love seeing things like a big Mario ice slide where you have to avoid ducking into the pipe when I've seen it in so many other hacks. Similarly, some of the music choices are a bit too familiar (e.g. castle with Filmore, like in GPW2).
I will say on the positive side that the game looks good and the obstacles are well-placed so it plays pretty smooth.