It is immoral to show things associated directly with sex. So in those terms, yes showing female nipple is.
In other cultures, female toplessness isn't an issue, and they find it humorous that westerners consider it to be so. Plus, nipples are not associated directly with sex, in fact, they really are entirely unrelated to sex.
On the other hand, in western culture it has been customary for quite a long time for women to keep their breasts covered, and they have gained thes status of "secondary sexual characteristics." Point being, its not appropriate in our society for women to show their nipples, but its not the same thing as sex.