From a food safety perspective, it’s perfectly fine (no pun intended) to eat them.
They are mostly fat, skin and a few fine ducts, so no real difference from the surrounding pork belly. Like with all mammals, if the female they came from never had babies or they came from a (usually castrated) male, the milk glands etc. deep under the skin won’t even be fully developed.
If the skin of the pork belly is removed for cooking or other preparation, the nipples will come off as well, sometimes you can see a faint circular trace of where they were, but most people won’t explicitly look for them.
The attitude of what’s “ewwww” and what’s “normal” has a huge cultural aspect - do you buy a whole chicken with head and feet and possibly even feathers or is the default nicely cleaned up and reduced to the rump with part of the legs and wings? Or do you opt for the clean chicken breast? And not all people like to be reminded that the neat slice of meat on their table is simply a part of what was previously a living, breathing creature. If the nipples bother you, cut them off, if not, you can simply let them be.
As with everything that’s rare (a pig has only twelve or fourteen tiny nipples on an almost 200 pounds body), they were once considered a delicacy and special treat. Heston Blumenthal used them for his Roman Feast: Pork nipple scratchings