*Warning, this comment contains spoilers if you have not played the game yet*
This game was a blast to play! This was a fantastic way of taking a basic platformer and breaking the 4th wall. A few things. First of all, for your first game, this was great. The game felt fantastic for a Scratch game, and it was higher quality than I expected. The jumps are a bit weird on slanted platforms if you're moving, but I think that's a scratch thing with bitmap and edges and it didn't impact my gameplay at all. I feel like the boss fight could have been shorter, as 8 lives for a boss who has pretty much the same attack pattern gets a bit boring, so maybe reduce it to 4 or 5 instead. The music doesn't loop perfectly on the final fight, but if you shorten the fight, most people probably won't run into this issue. Not a perfect fix, but in my experience, Scratch doesn't do well with music and sounds in terms of timing. As a ending side note, it's cool that you put your project file as something people can download and look at, but it does mean that people can just steal your game, so you may want to remove that.
All in all, good game, keep creating, and I hope to see more cool things from you in the future!