I am a constant complainer and have been since childhood
I honestly have to have people point out when I am over complaining because I had heard myself so often and it was a habit that I couldn’t self monitor.
Your mom is likely having the same problem and probably doesn’t see it as complaining but rather as telling you/making conversation. That’s what I used to tell people

I got lucky and a brave matter of fact friend said no you can tell me about it without complaining. And she was right but it really took an effort on my part to recognize and then stop myself and switch gears.
You could try matter of fact tell your mom one time that her friends might have stopped reaching out because instead of telling them about herself and life she was just complaining at them. If your mom gets really upset drop it and leave if need be.
she will reflect on what you said even if she doesn’t admit to doing so.