depends on which fighter. A guy with an iron chin like Dan Henderson would never lose. A normal guy with no training can knock out a trained fighter with a clean hit unless the guy can't be knocked out. In grappling an untrained guy loses 100% of the time, look up Franklin submitting a random guy in like 6 seconds. The thing about grappling/subs/wrestling is if you have no clue what you are doing you can't do anything unlike striking where you can throw haymakers and pull off a miracle knockout
Tito Ortiz got in a street fight with some untrained guy and got knocked out. I'd say with mma gloves on against an average UFC fighter an untrained guy could win 1/1000 unless he is Hendo or Rampage then it's 0/1000.