That's actually terrible for realism. It's not actually a disabling blow. In true anything-goes fighting you don't go for something like that as it is quite possible to fight through the pain. An elbow strike to the temple, a palm strike to crush the nose, an eye gouge, or something like that would be more realistic. Throw something truly disabling and not allowed in MMA. Still not super realistic, as even if the MMA fighter is not expecting it, reach would make them harder and the trained fighter is likely move to block/avoid even illegal hits. They are the kinds of things that let a smaller/weaker fighter beat a stronger one though.
Source: I've done about a decade of martial arts, of various styles, and done limited work with survival fighting, weapon defense, etc. Not professional or anything, but at least proficient.