>How come yall aint doin any gardening?
>you will never garden with applejack
it adds a visual element to the joke, which will undoubtedly make it easier for the audience to understand the joke.
a suitable alternative may have been for AJ to whip a fork or trowel from underneath her hat, or something like that. however, i'd imagine they were also trying to imply that AJ didn't quite understand why fancy ponies at the party wouldn't want to get muddy. which is the joke.