We all know Lukaku has a limited game in many respects, but I think anyone would be stupid to say that his finishing isn't great. Give him a good chance in front of him and there's a very good chance he'll put it away.
If our wingers and creative players were good at supplying chances this season then Lukaku would have scored a lot more. Wether you put our creative players being poor down to bad tactics, management, inexperience, etc is down to opinion but I think the bulk of the blame should lay on our manager\creative players for not giving Lukaku enough chances. He needs to work on other parts of his game badly, yes, but lack of good supply is the main reason he's not done well this season, not poor finishing.
If Lukaku improves on the weaker parts of his game he'll become a top drawer stiker. If he doesn't he'll still be a really good\dangerous striker, providing he's given good supply.