This season in the PL his goal conversion rate is 23.1%, which is better than all of Aubamayang, Vardy, Lacazette, Aguero, Firmino, Kane, Rashford and Higuain.
The key stat with Lukaku is that all of those players bar Higuain have had more attempts on goal than him this season and last season the likes of Kane had chances at literally twice the rate Lukaku did.
For example, someone like Aguero peppers the goal with attempts (even low percentage ones) which sees him miss an awful lot more but results in more goals even with a lower conversion rate:
Goal conversion: Lukaku 23.1%, Aguero 19.6%
Goal attempts: Lukaku 52, Aguero 97
Min. Per Chance: Lukaku 35.7, Aguero 26.
So your impression of him is entirely wrong. He is actually quite a clinical striker when he has a chance, he just gets/takes relatively few chances (either as a result of the team or his own play) which means any misses stand out. Whereas other strikers miss a lot more but the sheer volume of their attempts on goal carries them. Other strikers are also more likely to compensate for missed "big chances" by converting one of their many low percentage attempts, at which point their miss is forgotten.