My theory is that Mad Dummy was originally a ghost, and for some reason, he lived in RUINED HOME with his cousin, Glad Dummy. Now, to understand why I say this, we need to trace back to Undertale, with the Mad Dummy in Waterfall.

"I am a ghost that lives inside a dummy." He says, then states that his cousin used to live inside a dummy until Frisk (aka the player) showed up and talked to this so called "cousin" he was talking about. This ghost may well be Glad Dummy, and since this is TS!Underswap, a game where not only does it swap the characters, and keeps their personalities, this could be the case.

So, what made them separate from each-other? Well, when this ghost found Asgore's training dummy, his cousin got jealous and split into Waterfall, this would explain why Mad Dummy is so furious in the game.

What happened to Glad Dummy? He may have found another training dummy in Waterfall, or whatever Team Switch will call it when the full game releases. This other training dummy was used by, obviously, Alphys. Glad Dummy would then proceed to find a new, peaceful life in the Garbage Dump, just like where the Dummy was located in Undertale. Hence why the Glad Dummy is not-so-furious compared to his cousin.