A PhD is a PhD, at least in the UK. They are all examined according to high standards. Always involving an external examiner from a different university. Often, PhDs in Russel Group are examined by new university scholars and vice versa. If someone gets a PhD, a certain standard should be there.
Surely, the ability to transform a PhD into SUBSEQUENT publications is down to the individual student and has no impact on the standard of the PhD itself. Therefore, mediocrity is a very weird expression in this context.