My mom is always telling negative stories or telling strangers personal things about my family what can I do about this?

My mother was the same way. Standing by her while she babbled about her life to perfect strangers was embarrassing and could be annoying. She was also very negative. She once told my cousin that our large, beautiful dream home was “a barn”. The idiot then told me he’d driven up and down our street looking for an actual barn. People with dementia lose their social filters. They no longer understand how to act in polite society. You cannot change their behavior. You can only suffer through it.

Most of the people my mom babbled to were understanding and patient. On a few occasions. I stood behind her, smiled and rolled my eyes. We went to the same stores and people realized she had issues.

I doubt you're going to change this about her. I guess the only thing you can do is make sure she has as little information to share with the world as possible.

If you haven’t already, check out Teepa Snow online. She is great. She teaches techniques to work with dementia patients.
This phase will pass. Most elders respond to hugs and kind words. If she feels you are critical of her it will make things more difficult.
They are very sensitive to being “bossed”. Especially by there children. You are forever young in a parents eyes.
Watch a few of Teepas’s YouTube videos and see if you can relate. She teaches to redirect their attention. Thank goodness, most strangers are kind.