Not that anybody will read it now that it has been beaten to death, but most would consider me to be a fairly insensitive male and EVEN I KNOW there are LOTS of things you don't joke with women about.
1) anything regarding their appearance. height, weight, skin, nose, hair, feet, butt, chest, how feminine they are (anytihg that cannot be changed too easily) ... have had maybe 20 girlfriends and none of them ever wanted to be "kidded" (your words) about their weight.
2) how smart they are -- nobody thinks it is cute when you kid them about being dumber than you or dumber than your last gf. If they are really smart they will just be annoyed and think you are threatened by their intellect and if they are dumb then they will feel insecure.
3) never joke with them about their "look" -- includes clothes, makeup, jewelry, car, house, whatever (things you can change). This is a reflection of them as a person. Teasing them about being a loveable slob is not taken lightly like some men are willing to.
There are some things that women take really seriously and an engagement ring is one of them. It doesn't matter what your take on the tradition of the ring is (like these jokers on letsrun), that wasn't what you were asking about. It doesn't matter that she posted it on Facebook. When it comes to an engagement ring, when the ring si presented everyone is supposed to "OOOOHHH and AAAHHHHH" and say "Oh it's lovely!"
Every woman knows this. Even I know this.
I am not going to pass judgment on you or what kind of person you are or your intentions (I don't know you so how could I?). But you asked the question: Was I wrong?
Yes, you were wrong. You never, under any circumstances, put down or make any kind of disparaging comments about a girl's engagement ring ... picture ... or in person.
She is 26 for god's sake and your niece!?! Be supportive and don't question the quality of her ring, which right now BTW is the center of her universe ... or at least it is a symbol of the most inportant thing in her life.
Last, her supposedly being OCD has nothing to do with her taking offense to your comment. I think most women would not want unflattering comments made about something that is obviously important to them and they think it is cool.
One more, why do you consider your relative's comment about your husband working in a grocery store to be "obnoxious", but your comment to be "just kidding"?
Why is some kidding about working in a grocery store considered obnoxious (why is working in a grocery store even considered a put-down?), but your kidding about the quality of the ring is supposed to be considered funny to everyone?