I think it is partly age, inexperience with the world and with some of the feelings she is writing about—a less mature understanding of consequences perhaps—the callousness of youth and if it is that which let JA imagine mothers starving to death, it is no enemy to the story IMO for it is wickedly funny. I think it is also that JA just had a very sharp wit; someone who felt the need for excessive sensibilities in life might read L&F and feel it is very unkind but I guess that can be said for all parody as it is poking fun at something after all. Even when grown, JA sometimes made quite mean remarks in her letters—of course she probably did not envision that her letters would ever be published and read by millions. :D

“Mrs. Hall of Sherbourn was brought to bed yesterday of a dead child, some weeks before she expected, oweing to a fright.—I suppose she happened unawares to look at her husband.” (To Cassandra from Stevenson, October 27, 28 1798)