We already have Cooking.SE, and I have noticed a number of grilling/smoking questions on there, but they are hardly common. Would a grilling/smoking/general outdoor cooking proposal be redundant or would it fill a niche that cooking isn't really suited for?
It would be closed as a duplicate. If there aren't more questions in this area on Cooking.se, go ask them!
I think that the argument can be made that a lack of barbecue-related questions relates to a lack of association with the 'cooking' umbrella. For example, a barbecue enthusiast asking a specific question regarding a specific equipment setup is unlikely to get a quality answer on a board that doesn't target subject matter experts. I'd imagine that the demographics and interests of the average poster in 'cooking' vary substantially from that of your average barbecue hobbyist. There is enough substance in the methods, equipment, culture, history, etiquette, etc that differ from 'cooking' to give it a shot. That's not to say that there cannot be overlap, which is not uncommon in the realm of SE sites.