There is a sense in which this story is a lot about 19th century America and the huge culture clash between the Eastern, settled part of America and the Western part which was still being settled and explored. The East was supposedly civilised, advanced and more cultured, whereas the West was theoretically more simple, less-refined, and because of this, more easily duped. In addition, because of this, the Easterners assumed that the Westerners could be tricked rather easily.
If we think about these two depictions, we can see that Twain is deliberately including these two extremes. Simon Wheeler comes across as the perfect stereotype of an American from the deep West with his use of the vernacular and his garrulous speech and the way that he speaks in a monotone, clearly identifying him as somewhat unsophisticated and unrefined. Consider the way that Twain presents his use of the vernacular:
Why, it never made no difference to him--he would be on anything--the dangdest feller. Parson Walker's wife laid very sick once, for a good while, and it seeemed as if they warn't going to save her...
The use of expressions such as "the dangdest feller" and "laid very sick" and "warn't" clearly help reinforce the image of Simon Wheeler as an uneducated Westerner. By contrast, the narrator, who is supposedly Twain himself, deliberately uses a much higher and more sophisticated level of English:
I have a lurking suspicion that Leonidas W. Smiley is a myth; that my friend never knew such a personage; and that he only conjectured that if I asked old Wheeler about him, it would remind him of his infamous Jim Smiley...
Note the use of words such as "conjectured" that help to juxtapose the speech and dialect of the two men. However, the joke is that in spite of Twain's supposed sophistication and the naivety and lack of education of Simon Wheeler, it is Simon Wheeler who well and truly dupes the Easterner, and, by extension, the reader, with his tall tale. Let us not be too swift too judge by appearances!