As already stated, your bag will flatten from your weight, and won't be near as warm- if warm at all- at those spots of compressed loft. A synthetic bag won't compress as much, but still likely will not be near as warm for your back/butt as on top. Down will probably be near useless under your body weight. You don't need a pad for cushioning, but do need it for insulation. Bottom line, if it is cold enough to really need a bag/quilt on top, you are almost certainly going to need something besides your bag under you. Pad, UQ, HHSS, IX UQ, something. Unless you figure out how to use your bag pod style, wrapped entirely around your hammock. A few have managed to do that quite well.
Pads will for sure get the job done. If you can keep it under you - some can- many can not, varies with person, hammock and pad. That difficulty is why many hammocks have double layer/pad pockets.