There are a few qualifications to make a Boss Monster:
Their souls persist briefly outside of their body after death.
They only age when their offspring ages.
Now, this brings up two HUGE problems:
Mono’s soul is a hard soul. Black, crescent moon shape with a singular helix swirl wrapped around it akin to a barbershop pole, the inside of the soul yellow from determination like a jawbreaker. If you killed him, his soul would still remain there. Just a lifeless soul.
Mono is incapable of procreating. He’s got no ability to create children. Destined to be the first and the last of his species. The folly of evolution.
In short: Monochrome can’t be a Boss Monster. He’s also too nice to hurt anyone, so expect a very long, boring fight because his defense is ungodly. Wail on him for hours and pray to god Paperjam and Omni don’t notice their son’s beating.