I agree with this analysis that she was joking, but being at a disadvantage having recently become a convict, she's the one who would most benefit from exercising common sense.
So do you think her common sense should have told her to keep her mouth shut? As an idealistic young American with no experience with the justice system and a firm belief that what was happening to her was wrong, I think her common sense told her she had the right to speak her mind and to crack a few jokes to mollify the terror she felt at having just received a federal prison sentence. I think that would be a fairly normal assumption for many or most people.