There is most definitely such a thing as a too-plump cushion. This is the kind that, when you sit down, you instantly move out the way. A cushion should be sink-into-able – plump, yes, because that’s what makes them inviting, but soft enough to gently give way when you lean in. It’s why we stuff (but not overstuff) ours with feathers, which happily yield but equally puff back up again.
Size is important too. Try not to have too many small cushions, whether it’s on a sofa or a bed, which can feel fussy. It’s the big, square cushions you want to dive into that provide cosiness ultimately, while littler ones are instead useful for supporting your neck and the small of your back.