One of my ex-GFs was a local amateur boxing champ and she literally beat up quite some average male amateur boxers, so people who maybe were not the highest skilled fighters, but definitly way more trained than the average Joe. So I have absolutely no doubt that any elite professional WMMAist, who's probably 10x the fighter my ex-GF was, would destroy your average Joe before he even could call for his mama.
And bringing the argument of the difference between a streetfight and sanctioned MMA match makes only sense if you compare an MMA fighter and someone who is trained in some other fighting art. But your untrained average Joe is exactly that: UNTRAINED in fighting. He doesn't know how to properly strike, nor is he used to getting punched and even less is he used to submission grappling. So why should he have an advantage in fighting skill over someone who is actually trained in some form of fighting, even if that is not 100% suited for streetfighting¿ Of course, any average guy can deliver a lucky punch, but on average the average guy definitely loses.