OP, I don't know if your daughter and her friend specifically are trustworthy enough to walk around the Mall of America, which is very big. It seems like this depends a lot on the kids.
BUT if your concern is based on the idea that sex trafficking of girls walking around malls is a real concern, you are just wrong about that. Crime rates of all types, including violent crime and crimes against children, are DOWN significantly from when we were young. There is a lot of attention paid in the media to the VERY rare instances where children are kidnapped by strangers, but it is extraordinarily rare. Your DD is much, much more likely to be killed in a car accident on the way to the mall than sex trafficked once she's there.
Here is some actual crime data:
http://www.pewresearch.org/fact-tank/2019/01/03/5-facts-about-crime-in-the-u-s/