I took my girlfriend and friend out to dinner.
Friend says, “I love living with someone.”
Turning to my girlfriend, she asks, “how do you feel?”
My girlfriend looks at me and then replies, “it’s…fine.”
What do you feel is an appropriate response?
I sat down with her later and tried to talk about it, but she said she was joking. I just don’t get the joke.
Without more context (how well does your GF know the friend? How well do you know the friend? How long have you been living together? Have either of you lived with anyone else in the past? What else was said immediately after this exchange? What else has you GF said to you about living together? Whose idea was living together in the first place? Did one of you live in that home alone beforehand? What is your GF’s sense of humour like otherwise? etc etc) it is going to be impossible to be sure about this, but living together is always going to involve compromises of some kind, so if this is relatively new for her, it is very likely that there is a grain of truth if she sounded like she has some reservations.
One thing that is definitely not going to help, though, whether it was a joke or not, is you getting all defensive about it.
She has told you it was a joke. I’d suggest telling her that even as a joke it has left you feeling hurt, and then ask her if there genuinely is anything that she sees as a problem and what she would like from you in order to make it better - and then commit to make those changes where that is realistic. And take the opportunity to so the same the other way around: what changes you would like from her - so that you are both communicating openly, and trying to grow and make the relationship better.
This is the response OP should read.