It's an interesting idea, and there's a lot of potential around it. Being she could have learned it from Manehatten, or wound up being taught it, though their are so few fics that actually use that facet of her character. How come? Is there a reason for it or no?

Conner Cogwork's Ah Ain't Got no Ack-cent! was one of the first Applejack stories I ever read, so they certainly exist.

That said I would imagine there are "so few fics" because AJ has plenty of other aspects to explore beside that one—like her parents, her extended family, her relationship with Apple Bloom, or the fact that the family farm sits on the edge of the Everfree Forest.