Babe, no city is safe in Mexico.
The only city that’s I can say would be the “safest” is Reynosa. I always go through there when I visit my grandparents. Matamoros is close to the beach, but lots of cartel drug trafficking.
People can easily tell if you’re coming from the “other side” how they call it, and will treat you different. Obviously there will be people out there who WILL try to take advantage just for the simple fact that you’re coming from the US. Trust me, I’ve been through that.
Be very careful and stay safe!! I wish you the best