Character mistake: Dumplin' is looking through her aunt's belongings, and finds an old application form for the Miss Teen Bluebonnet pageant. The application has an August 1993 signature but marks Lucy's DOB as 5/5/79 with a stated age of 16: it does not add up, as she'd have been 14 instead. Granted, it's an application she never actually turned in and not an official document, but this does not justify the mistake in the context of the movie (not to mention that had she just been too young to participate, the point of the movie would be moot).
Other mistake: Willowdean is disqualified for not having her talent costume approved - but the girl who gets 2nd runner-up [Millie) is also wearing a different costume than the one she had approved, and she is not disqualified.
Continuity mistake: As the young Willowdean puts on the table the Dolly scrapbook for the party before the doorbell rings, in the closeup she's running her left hand along it with the pinky and ring finger off the side, while in the previous and following shot she has her hand fully pressed to the cover.
Continuity mistake: At the beginning of the movie, with Dolly Parton playing on the car radio, we see the nose of the car driven by Aunt pass by a red Kia with a bunch of girls doing their exercises running on the sidewalk. A few seconds later though, from the passenger window we see they are passing by the same car and same girls while the song is playing and they sing along to it.