I think it depends how much the original content has been altered. I often use sims from the gallery as a base for my own sims, but the end result looks so different that I don't really see the need to credit the original.
Similarly, if I download a sim from the gallery and they have a child with one of my sims I don't feel that I need to credit the maker of the gallery sim if I upload the child, even if they look very similar to the original sim I downloaded.
With lots it's a bit different, but I think it depends on why you uploaded it. I upload my lots because I want people to play them and enjoy them, so if a steal got more likes than mine I'd honestly be more happy that more people were playing it than annoyed at the theft, though I would still be annoyed. The amount of editing also factors in. If someone completely re-decorated one of my lots and it looked significantly different I wouldn't mind so much.
