It's cute. The first world didn't do much for me in terms of level design, but this seems like one of those hacks where it gets into more interesting ideas later on. I'm not sure how I feel about secret exits not being indicated on the OW - personally I don't find much enjoyment in meticulously checking every spot in every level for potential secret exits. I'd rather just enjoy the platforming.
The presentation was nice, especially the crisp palettes. I wish the soundtrack consisted of something other than horribly overplayed SMW remixes though.
I'm glad that you got someone to code custom bosses for you, but I'm not sure if they fit in their respective worlds, difficulty wise. The world 5 boss was the easiest out of all of them and the world 1 boss was the hardest. You might also want to work on the telegraphing for the world 1 boss. In the third phase, it's basically impossible to tell where he's gonna go next.
Also, at first I though the tiny flames on the floor in the world 2 boss battle would hurt you. I think you should get rid of them.