Despite being poor by his standards he's still ended up with 27 goals + assists for the season. That's not a bad record considering he's had some important moments in the UCL later stages as well.
This season has been a pure anomaly in terms of his finishing, yes he has been inconsistent in the past but never this bad. I think it's a case of mental fragility and not being a poor finisher because he's never been this bad in the BL, where he gets the same type of chances.
Would be interesting to see what happens next year, if Chelsea bring in another striker and, if they do, how will Werner react. I don't think he will do much better if there is a striker playing ahead of him.