A kind, gentle, and highly maternal woman, Lady Jane humanizes her husband, Pitt Crawley, appreciates the domesticated Rawdon Crawley, and eventually quasi-adopts Rawdon and Becky's son.
We don't see that much of Lady Jane, but what we do see is awesome. She is super nice to all the rather difficult members of her family, generous with her time and energy, loving and protective of her children (and eventually, of her nephew Rawdon Jr.), and generally an all-around wonderful woman. On the other hand, perhaps that's why we see so little of her. In a novel where all the important and memorable characters have major flaws, someone as well adjusted and normal as Lady Jane just doesn't have that much of a place. The novel thrives on mockery, and there's just not all that much to mock here.