“My fiancé is jobless. It’s been four months since she had her mom be a joint sponsor. Do you think it’ll work?”
It comes down to the income of the mother. If her mother’s income is solid and stable, then it’s not going to be a problem. So, that’s really what it comes down to. Also, make sure you check whether your fiancé’s home country is one that the U.S. allows joint sponsors for.
Your fiancé could have no job at all, that’s perfectly okay if the joint sponsor, in this case, the mom, is suitable. So if she has a solid good income, that covers her household and you. If, according to the income requirements, she has adequate income and all the evidence required to prove that, then yes, you should be fine.