Basic Trope: A character's outfit looks like it was assembled by the Random Number God.
Straight: Dave's everyday clothes are really mismatched and not very practical.
Exaggerated: Every single person in the setting has not one, but several costumes in his or her wardrobe, each ridiculously overblown.
All outfits are mismatched because the story is set After the End or During the War, when new clothing is in short supply; or perhaps protagonists are just too poor to afford many new clothes. They have to get by with what they have.
Dave is lazy, and either doesn't care to think about what items of clothing will go with what, or can't be bothered to wash his clothes most of the time. So he just throws on whatever he can pull out of his wardrobe first.
Dave is part of a subculture which values a mix-and-match approach to styles. It's a deliberate fashion statement.