"oh no, someone on youtube does a thing I don't like to make their channel more fun, better type out an angry message on an unrelated site to show everyone how my taste is superior to everyone else's"
but on the "armflailing"thing, it's a muscle memory trick to help him find the pitch. Most musical instruments have at least some physical controls on the surface to produce different pitches and using them taps into muscle memory. Since activities like humming, singing, and playing instruments that don't have any physical controls (kazoo), him moving his arms around makes his brain associate that particular pitch with the arm movement.
plus the video would be super boring to watch if the whole thing was just 8 copies of him sitting there deadpanning the camera.
that's cool. I'm not saying you aren't allowed to not like it. you just sounded like you were trying to start a "I hate this and no one else is supposed to like this" red thumb circle jerk that FJ seems to like doing so much.
and yes, he does over do the arm thing but again, it's to make it visually stimulating as well as nice to listen to.
I think the song is a play on megalomania, the obsession of having and obtaining power. It may play during Sans's fight, but it's about YOU, the player. Not Frisk, Chara, Asirel, or even Sans. It's a personal message about you. You killed so many to see if you would get more powerful, only for the game to call you out on it. The whole game is watching you, and if you go down the path most RPGs do, you're going to have a very bad time.
This is why Undertale is easily my favorite RPG now.
My first System was a Nintendo. Some Game were stupid hard to beat, and watching videos of this game's "Sans Fight" makes me want to play just for this battle.