Yes, it is very hot, but it depends on what your tolerance is and if you have dressed appropriately. I and many others were outside watching professional tennis from 5:30p on in Grapevine last night, not to mention the players on the court. So it is doable, it is just whether you are willing to get very hot. You might want to bring an extra set of clothes to change into after you get back to the airport. (Or if you are wearing pants, bring shorts to change into on arrival, and then switch back to pants back at DFW.) As to whether Grapevine is worth it, I don't know, but personally I'd almost always rather spend some time seeing something as opposed to just an airport.

You could just walk around the outlet mall instead of downtown Grapevine. It isn't an airport and it is inside.