I think when the obvious fault in a team is that chances aren't being taken, then the first-choice striker, as supposed main taker of those chances, is always going to be a scapegoat.
And
this article from Sky Sports doesn't exactly help his case. It's from the end of October, but talks about how he's missed more big chances than his Premier League striking rivals. I'm not sure how I'd go about finding more current statistics though.
That raises the question, would Martial or Rashford have converted those chances? Maybe, maybe not. Would they have more or less than Zlatan's 13 goals if they were the main striker? I think his lack of pace and poor passing has sometimes been an issue, but then he sometimes does things that I don't think either of the other two would even think to do.
Personally, I'd keep the faith in him. As dwd says, 13 in 22 isn't bad, and I've a feeling that things are going to click for him.