If something is smuttier, it has more dirt or soot on it. Also, if a book or movie is smuttier, it has more sex in it.
The word smuttier can make people think of things that are not clean or things that are not good for kids.
Examples:
The smuttier window was hard to see through.
This room is smuttier than the other one.
The smuttier the room, the harder it is to breathe.
I don't want to read that smuttier book!
The smuttier the movie, the more my parents won't let me watch it.
The smuttier the window, the less light comes in.