Ok, I’ll admit I don’t know what “lumpen mesomorphs” are but for some reason, it makes my hackles rise.
There was a time when it was theorized that personality was related to overall body structure, and there were three body structures: endomorph, ectomorph and mesomorph.
“Mesomorph” was synonomous with lard-butt (the other two are “lean” and “big, well-muscled, but not fat” - I don’t recall which is which).
And “lumpen” is generally synonomous with “lump”.