Humorists such as Mark Twain make use of exaggertion. Certain exaggerated details and incidents are described by a storyteller in a very serious tone, a tone which makes the story even funnier because the reader gets the impression that the teller is unaware of the ridiculousness of what he is describing (i.e. satire).
The regional dialect is an important element of Twain's humor and use of the ridiculous. These dialects capture the local color and make the characters more interesting and amusing. Simon Wheeler makes for such an interesting character, and his use of unexpected words adds much to the humor of the story. For example, Simon relates, " This-yer Smiley had a mare--the boys called her the fifteen-minute nag...." Such colorful language creates humorous images in the mind of the reader.
Many times, like in Mark Twain's work, authors use regional dialect and even phonetic spelling to add to the authenticity of a piece and its overall impact. Authors purposely choose their setting and characters to reflect and emphasize certain themes, symbols, allusions, metaphors, etc. Think about the novel/film "The Help." The setting is 1950s Jackson, Mississippi, at the cusp of the Civil Rights movement. The characters are upper-middle-class white women and lower-class black "help." It would be very difficult to understand the nuances and racism of this culture, along with the emotion and power of characters like Minnie, if the author hadn't used regional dialects!