In the first scene at the station, two hicks are discussing "Mickie Mantelo" and his home run career, and one is boasting how "Mantelo" hit one almost 250 feet. The average distance to the outfield is 320 feet - almost 70 feet short of the outfield fence.
This is not a slip up!: The two truck stop employees are talking and one of them, as we find out later in the film, has a kid on a little-league team. The one guy says jokingly to the other guy, "Did you take 'Mickey Mantle' over to the baseball game?" He is saying "Did you drop your kid off at the game?". And I am sure that that distance is a lot on a little-league field.
I own the movie and am watching it right now. This is how it goes:
Joe: "So, did you get Mickey Mantel over to the baseball game?"
Duncan: "You can joke about it all you want...but he hit 3 balls over the fence and one of them must've gone 250 feet."
Clearly Joe is joking about Deke, Duncan's son, who is in a little league baseball team who we later see. I hope this clears it up.
The average distance to the fence might 320 in MLB, but this is little league, where the fence is 200-225ft away. So yes, the kid could hit a home run that went 250ft. No slip-up here.