The opposite of monstrosity is something that is beautiful or good looking.
Some examples of how to use monstrosity in a sentence are.
The new hotel is a monstrosity that blocks my view of the beach.
Because your monstrosity of a car is parked in front of my driveway, I can't drive to work.
Nobody will purchase your couch at the yard sale because it's a brightly colored monstrosity.
That big ugly factory is a monstrosity that they build close to the beach.
Monstrosity means that something is very large and is an eyesore.
For example a large building that is ugly could be considered a monstrosity.
A monstrosity is something so hideous that you can hardly stand to look at it, like a zombie or an ugly modern building.
A monster can certainly be described as a monstrosity, but so can anything that is as frightening or disgusting as a monster.
Basically anything you find large and offensive to look at can be considered a monstrosity.
A statue, a person etc can be a monstrosity.
Since monstrosity is just anything that is large and is terrible to look at.
Although what one person finds ugly and a monstrosity someone else may find that same thing a beautiful thing to look at.
Beauty is in the eye of the beholder.