I think Sterling gets so much stick for his finishing that it sticks in our minds, and becomes accepted fact, but I'm not sure he is that bad. I think Aguero is the best striker the PL has seen, yet if you had a drinking game for every time he missed and the commentator said "that's a collector's item" you'd spend most matches drunk.
I posted some stats recently in the "striker" thread, comparing his shots per goal with Kane and Ronaldo over the last three years (so that includes last year, when many thought Sterling was really poor). He came out top, however you sliced it. It's not a perfect comparison, but it does suggest he's not as wasteful as many imagine.