I'm a little late to this one, but I've noticed that different people have different ideas of what is considered "appropriate" PDA and what is inappropriate. I would have been very uncomfortable, even resentful, but it's mostly because I'm single and I feel like I'm always on the outside looking in at something I can't have. That is my problem and no one else's, so if I see this kind of behavior I simply find something else to pay attention to, like my shrimp cocktail or an interesting conversation with a friend at the other end of the room.
I don't know if it was appropriate or not. I think it depends on how casual or formal the gathering is. Since it's your mom's home, she can object to something that she feels is inappropriate. Venting to someone about something that's upset you might be a good way of letting off steam, but it doesn't do anything to change the situation. I think that, depending on the relationship she has with Zoe, it would be fine if she had a discussion with her, just the two of them, and said, "This made me a little uncomfortable, and I would appreciate it if you didn't do this at my house or in front of me."