Don't confuse the greatness of Jamerson's lines, the great singers and the super successful hit singles with the majority of Motown's output. Motown put out a lot of unremarkable songs on the albums which were intended to be throw away pop music. Motown was always well written, arranged, recorded, performed and played but most of it was very forgettable.
Jamerson wasn't unappreciated by Berry Gordy and during the Detroit years I believe he became the highest paid session player in the US. I think Jamerson was the most creative and influential bass player. He didn't just open the "busy bass lines" door for the rest of the world, he knocked the whole wall down.
In spite of JJ's unique greatness I still prefer Tommy Cogbill with Aretha Franklin, David Hood with Mavis Staples, Duck Dunn with Sam and Dave and Vernie Robbins with Jean Knight.