I think it is very difficult.
Obviously I don't know your situation. I don't think that anyone should underplay the illness of another person. It shouldn't be a competition. No, your mum shouldn't be insisting that she has it worse but then I don't think you should be telling your mum that she has it better than someone else.
I don't know, unless you've walked in someone else's shoes I wouldn't be too quick to judge.
Chronic illness, no matter what it is, is very difficult to come to terms with. Your life is limited and impacted in ways you'd never imagined and that isn't easy to accept.
There isn't a right way to deal with it, just as there isn't a right way to grieve. Some people are very accepting, others get very angry, others self destruct. You can't tell someone how to deal with it. I guess all you can do is remove yourself.