I think you know your mum best, so you know if it's likely to be attention-seeking or if there's something deeper going on like health anxiety.
My mam had leukaemia back in 2018. She got treatment and has reached her 5 year survival last year. She does still bring it up regularly, and often to people she's just chatting to out and about. She can have some attention-seeking behaviours, as well in other ways, too. So, I do attribute some of it to her attention-seeking.
However, whenever I find myself getting irritated, I take a step back and remind myself what it was like back then. What an absolute punch to the gut it was for all of us, especially my mam. How terrified she was and that her first question to the doctor was "Am I going to die?"
That kind of stuff stays with you. Those fears linger, even as the years pass. I remember when she was going through treatment, my mam crying and saying to me that she just wished she could get a new body. She was sick of being sick and living in fear.
I think I've realised that even though it's "over" and she's doing exceptionally well, the issues that having cancer caused still linger and her little comments are her way of letting out those issues. It can sometimes be frustrating because I sometimes just want to forget all about that horrible time in all our lives but then I try and remind myself that whilst it was hard for me and the wider family-it was a nightmare for my mam and a nightmare she couldn't escape from.
It could be that your mam just needs to talk out these issues and feel safe exploring her fears. It's possible that she may feel that it's taboo to discuss these kinds of things with her family-but the anxieties still linger and this is how she shows it. I'm always living with the temptation to try and sweep it all under the carpet-but it's not my burden to do that with.
I try (and often fail) to remind myself that just because something is technically "cured" it doesn't mean that the psychological damage has been cured or dealt with. Maybe speak with her, one on one, and try and let her have free rein to say what she feels. Don't try and remind her that she's cured. If it is anxiety, logic often won't puncture that. She needs to feel heard and listened to. Maybe she does need some counselling.