Don't zoom in on your characters or the quality just rektz the screen. The walking animation of Fisk takes you out of it. If anything in any animation, the first rule is to keep the quality consistent.
Personally, I don't know if it's that "Stronger than you" parody guy who started it, but everyone gives Sans a deep voice. I don't get it. It just seems like it's somewhat less the creator's idea of his voice and more of fad around the fandom.
Personally, I have great ears and decent eyes, but I don't work in audio enough to say what others are saying about it. So no comment there.
If anything, I'd say make the music toned down or show less of the fight or more of it. There was no real build up nor surprise to Sans's death the way the music is edited.
Everything is action and reaction, push and pull. If it's rushed, then rush it. If it's not, then don't let it be. Keep in mind this is after seeing it for the first time.
If you get to be great, work on first impressions. I think that is an animations final milestone. Looks good besides the pixels personally.