I get the strong impression Big D was absolutely livid with her results and that’s why she’s so obsessed with them. Her whole life seems to be oriented around the contradicting goals of impressing him while also trying to prove a point by “taking her own path”.I've always assumed she got 4.5 C or higher GCSEs and 2.5 D-G which the school and her parents and thus probably herself considered failures.
Maybe Mama E was displeased too, maybe she was supportive who knows - Jack doesn’t seem to have anywhere near the desperation to be approved of/prove wrong/get attention from her as she does her father.