funnily enough, i have an outlandish theory in which mad dummy is mike. it stemmed from the mad is spamton theory. but that was disproven with the bonus room cut scene.
i do think mad has ties to spamton, (i would explain all the correlations, but I am vastly too tired for it)
but. mike- or at least the person handing out shadow crystals- has to be a lightner. the knight has one, and asgore has one.

Asgore and Spamton both have similar ties. They both know mad. In chapter 2, mad makes a flirtatious comment towards asgore. so mad knows, and would have a drive to try to interact with asgore. Mad also is able to interact with spamton neo by helping mettaton create the vessel. In doing so, they left the shadow crystal behind.

This theory very much has a lot of random leaps, and the fact that Mad is my favorite undertale character (i need more mad deltarune content)
But, I also do think that, if this theory is correct. Then Mike is a misunderstanding. The characters got the spelling wrong. Spamton heard the name over the phone. Tenna heard it from spamton. A game of telephone if you will. Mike is spelled Mic. And it’s an acronym for something. Mad Incorporeal Copy or something.

But, Mad being another Cat Girl is often over looked in many theories (mad mew mew my beloved)