Not being a MasteringEngineer I'd say,
that term you used would not be the way they see themselves,.
Not a repair job ideally.
The job of a person who tonally enhanced and engraves.
If the mix engineer is a total tweaker,
the mastering engineer is a refined tweaker.
Give a mixer a year to mix a song, he'll take a year.
Give a mastering engineer a year to master a song,
he'll master your song 365 times and return one copy.