This X.com post explains why Mikoto is called "monster". https://x.com/itokonoko0804/status/1924299006097887314

Thinks it’s a little rude, but accepts it since it’s also funny. ->

“Monster” because
* She’s too high spec. (conventionally attractive)
* You can’t tell what she’s thinking.
* She can calmly do things that you normally wouldn’t be able to.

…that’s her nickname around the area.