It's kind of hard to look at the Mousasi fight with any real critique. A fight with a late replacement opponent, with a completely different style, who he has nothing to gain by winning or losing to, is not a fight you look at too seriously.
Mousasi probably could have gone out and smashed the guy, but Mousasi basically got a freebie where he could get actual octagon experience. He made the right call. I'm not saying he let the clock run to a decision, but not being in a rush to finish wasn't the worst move he could have made.
Only thing you can really say is that he won and now has a win under his belt in the UFC, allowing him to be comfortable in his next match.