Lumpy means covered with, or full of, small solid pieces. Lumpy sauce has bits in it that did not mix. A lumpy mattress has places that stick up, so you cannot lie flat. A lumpy bag of sugar has hard clumps from damp. The word is ordinary and a little informal — home, cooking, complaints about beds and roads. Though actually, people stretch it: a lumpy path, lumpy progress, a lumpy year, meaning uneven, not smooth, with awkward bits sticking out. In cooking it is often a fault (you wanted it smooth). On a sweater, lumpy might just mean thick and bumpy, not wrong. It does not mean 'a lot of' or 'heavy', only that the surface or the mix is not even.