People speak variously. Some talk in clichés, some talk like the authors of philosophy tomes. Some people slip back and forth between dialect and formal language. Writers can signal that they know that the character’s speech is idiosyncratic:

Proget was a weird mix of garret and gutter. One minute he’d be holding forth about post-modern discontinuities and the next he’d be asking if you noticed the receptionist’s bazooms.

The result is that readers accept the peculiarity of the speech and, in fact, see it as a distinctive feature that helps establish the character.