To me "run of the mill" meant something was just average in quality. Here is what Quinion had to say:

nother phrase he mentioned that sounds as though it originates in the corn
mill is run of the mill, meaning 'undistinguished; ordinary, average',
perhaps referring to the ungraded output of the grinding process. But as the
miller ground in batches whatever came to him and gave the resulting flour
back to his customer, there was normally no question of separating it by
quality. In fact, the expression is modern (OED2's first citation is dated
1909), and it seems to refer to the unsorted output from a textile mill.
Another form of about the same date was run of the mine, with a similar
sense concerning ore.