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nobody cares about your mum, people are making jokes about the comedic construct of 'your mum' (entity unto itself.)
I recognise that this may not make this any more pleasant for you, and also that this veers dangerously close into the 'racist jokes are fine because I'm not racist' excuse, but it is important and hopefully helpful to recognise that neither you nor your mother are being attacked.
Then I think both of you need to be more careful about making such comments, not everyone has a mum anymore to be laughed at or be made light jokes about. I repeat what I said, if you don't know me or my mum and this applies to anyone, be careful what you say. Not everything should be joked about.