The idea is not bad, but the execution could've been better. I liked the cuts, although I think there were some too rapid cuts back and forth. Sometimes you could've stayed on a character (like Bubblegum singing and gesturing) instead of cutting away elsewhere right away.
The fact that there was no background to relate the characters to made these cuts even more confusing (on top of making the animation look kinda cheap)
Both the drawing and the animation quality was pretty uneven. I don't know why the singing Bubblegum near the end is so well done but the dancing Finn is so bad. I don't know why the crying Flame Princess at 00:16 is so well done and Rainacorn is so bad.
The lineart was also uneven. On top of mixing hand-drawn and computer-generated lines, the ones with the computer-lines looked very unnatural, because all line thicknesses were even. Try to play around with the variable line thickness and they would look much better, make them look more organic while still being precise (or stick with hand-drawn lines).
Some lines were also out of place, not ending where they should've ended (see Bubblegum's hair in the three-quarter view)
And lastly: I really liked that you took the time to lip-sync to the song properly, it looked nice.
I'm only going to give it two stars, but I'm sure you're on the way to improve.