Twain made use of the vernacular in this and in other works of his fiction to add an element of authenticity to his work. Clearly, having the dialogue that the characters speak set down as they actually spoke it as far as possible gives us a real insight into the sound of their voices and their way of speaking and view on life.
Mark Twain was deliberate in his writings. His writing reflects imperfect "exactness". Always remember one thing when reading Mark Twain, satire. He wanted the reader to be there in the story. He wanted the reader to think about what was going on and relate to the characters.Whether the character was a frog, a boy on a raft on the Mississippi, or a Connecticut Yankee, for Twain the reader had to be able to get "inside" the story to "get it".What is interesting is that he knew there would be some readers that just didn't "get it".Twain did not take this as a fault on his ability as a writer, but smiled because it was a fault of everyone else for not "gettin it".
Twain would use colloquialisms (local speak) to maintain the accuracy of the setting, so the reader would feel like he or she was actually there. If it were written in standard English, then it wouldn't reflect the historical time period or local setting.
To add to the characterization. Are the "mistakes" lending themselves to the character? DO they give you a feel of what the character or County is like? If so, ht eauthor is using these mistakes as style.