Too many clothes does NOT make one colder. Restrictive clothing makes one colder.
It does not matter whether your insulation comes from more clothing or more down in a quilt, it provides more insulation. However, not using that insulation correctly can have negative effects as is the case with clothing which restricts blood flow. As is the case with a quilt with extra down pulling it away from the hammock bottom.
A snug quilt and loose clothing will be warmer than any combination of of less optimal conditions.