OP, Gosh I really feel for you. What an awful thing for you to hear, esp when you are so far away. It’s certainly possible that she is suffering from depression Or maybe it was triggered by an especially awful fight. Do you have siblings or close friends or family there who could check in on her? It sounds like something someone would say to get attention, assuming it’s not related to depression.
My mom Suffers from anxiety and depression and it is a very difficult thing to go through. It’s possible that your mom’s relationship with your dad triggered this, but it’s hard to say. I think in that generation in a patriarchal culture, our moms put up with a lot.
When you talk to her, you can reassure her that you need her to be there and there’s a lot to look forward to, and the grandchildren are still young and there will be graduations and birthdays and weddings. You can ask her what made her say that and let her know how it makes you feel. Do you think it would be helpful to explain to your dad what’s going on? I feel like he needs to know.
"Friendship is born at that moment when one person says to another, "What? You, too? I thought I was the only one." C.S. Lewis