I have no idea what the reasoning might be for using a mixed breed. Any of the dual purpose breeds will work for a meat bird, but you'll get the most meat from a cornish cross or similar from a hatchery. They have the most meat per feed and time of anything else. If you want something sustainable, that you wont' have to buy chicks for every year, any dual purpose breed should really work. The cornish crosses and such don't because, among other harder to explain reasons, they are hybrids and their offspring wont' breed true.
Edited to Add: I'm assuming you mean barnyard mutt when you say "mix breed." If you mean a cornish cross or freedom ranger or whatever by that, then it's the best choice because those have been bred to give you the most meat for the amount of feed and time you put into them. They are like hybrid tomato plants: That particular cross whose parents have been developed for years will make what you want, but the offspring of that cross will not.