Are you not classing "Only in the Pan Handle." as a joke?
It wasn't a joke, it was a stupid thing for the POTUS to say, but he meant it as a joke.
The audience member made the comment about shooting people crossing the border, the audience laughed at the deliberately inappropriate comment, Trump acknowledged it with a jokey reply.
Neither the audience member of Trump is advocating mass shootings of brown people within the US. Trump specifically stated his opposition to using weapons (implying lethal force) at the border and an audience member replied with... well what you've seen. Even if the audience member legitimately did believe that border security should shoot people they spot attempting to cross illegally that still doesn't imply support for a random mass shooting within the US!