Well it was lucky in the sense that it knocked him out the way it did, and ended the fight in Marquez's favor when pacquiao was clearly dominating otherwise and had Marquez cut and swollen.
It wasn't lucky in the sense that it took no skill. In fact, I think that was the type of shot that knocked manny down earlier in the fight. Def a skill shot by Marquez, wherein he lured manny into it and went over the top.
Manny wasn't being careful. He thought he had Marquez and was trying to finish him off. Just ended up getting caught cleanly.