I'm curious which Mingus sideman you are referencing as making mediocre recordings.
If I think about the recordings of Mingus sideman (Eric Dolphy, Jaki Byard, Roland Kirk, Booker Ervin, Gene Shaw, John Handy, George Adams, Don Pullen), they comprise a disproportionate number of my favorite jazz recordings.
In addition, virtually any Ellingtonian sideman session is virtually guaranteed to be of high quality.