It sure as hell didn't help him, but i dunno if it was the decidin factor. JMM got a 2nd wind in the middle rds, and he was killin the body.
JMM was able to put the fight in the middle of the Ring after the cut. And Diaz can't do anything after that.
The body shots were the deciding factor.
It did set up the first KO punches but only when Diaz was cut and doesn't know what to do next. The next round Diaz tried to close the distance again but the damage was already done to him and he hasn't recovered yet, you can observe it in the way he punches.
Nau I am a big Diaz fan, and think he was on even terms at the stoppage, but Marquez was coming on previous to the cut.
I still have to say though, that Diaz does have some sort of psychological block when it comes to getting cut, and doesn't know what to do when it happens.