I would classify something as NSFW when it runs the risk of creating a hostile (in the sense of discrimination, sexual harassment, etc.) environment for others you're interacting professionally with - coworkers, clients, customers, etc. I don't think the poop emoji fits this criterion. It's potentially gross, juvenile, etc. but not hostile. This may vary by culture and context, of course.
Now, whether it's suitable/good UX, which seems to be what you really want to know, is another matter, and really depends on your application's audience. The answer by cloudworks gives a great example of a unique situation in which I think it is very appropriate. Apps with casual/lighthearted purpose, which people interact with purely by choice, not because it's something they need to use to get work done, would also be candidates where you could consider its use, contingent on cultural considerations. In most contexts, though, I think it will just, well, make your app look like 💩.