I'm easily irritated by this because I'm from the south. If the dialect isn't right, it rings untrue in my ear and it's hard to feel any connection to the story or the author. I quit caring about the character too, the story no longer matters. It's especially offensive if the character speaks like a cartoon character instead of a real person. If a writers judgements and prejudices shine through, forget it. I'm out.
According to this Literary Vocabulary website:
Dialect is the language of a particular district, class or group of persons. The term dialect encompasses the sounds, spelling, grammar and diction employed by a specific people as distinguished from other persons either geographically or socially. Dialect is a major technique of characterization that reveals the social or geographic status of character.Dialect is different from accent. Charles Carson, managing editor of the journal American Speech defines accent as referring to how a character pronounces their words. Whereas dialect is about what words are used, how the words are pronounced and how the sentences are constructed (emphasis is mine).
Margaret Mitchell's use of dialect in Gone with the Wind has long been a topic of discussion. Mark Twain's Huckleberry Finn is a classic example of authentic dialect.
What are some of your favorite examples of dialect?
For more on characterization, check out this Character Chart for Fictional Characters.