There's a flashback about it.
Mewmans pretty much treated Solaria as a mad queen because there was nothing like a "monster menace". In fact monsters usualy just made noise around their stolen land, in a pathetic attempt to scare everyone away, which never worked.
Solaria was so blood thirsty she made a huve deal out of this and was willing to recrute soldiers for a war but everybody ignored her crazy ass. Mina just happened to be a super poor pathetic and ugly girl nobody loved, so when Solaria showed fake care towards her she joined her army.
Solaria repeated the process with other hungry and loveless individuals but noticed they would ran away from battñe out of fear and how senseless it was. That's why she experimented with them and erase all trace if fear or pity of their minds.
Solarian Warriors literaly live in a constant nightmare in which they can't stop hurting others. Either they died by the monsters, killed each other or themselves.
Mina was the only survivor because her mind got so broken she just accepted everything Solaria said... but the problem is her mentality can't change and neither can she learn. She is stuck.