I'm going to tell my mom to stop complaining
Why do you have to tell her she should stop complaining?
Because I know she'll never change, but she should.
Although it is understandable that you would find her complaining annoying, it would be rude to tell her to stop. maybe you can think of a more positive way to frame it. Maybe you can think of some way to encourage her instead of telling her to quit complaining.
So how am I supposed to do it?
Try to see the root cause why she's complaining and see if you can help her stop the behavior or maybe try and find a way to make her endure something better and make her think that it is a better and a more rewarding experience rather than always complaining all the time.