Crowder's had an awesome season.
I suspect his shot will be up and down over time. I don't believe he just suddenly turned into a 18-20 point per game scorer. His shot this year has been better than I ever reasonably expected it to be. The Celts don't need him to be a deadeye, though, they just need his spot up jumper to be a weapon other teams respect.
Similar to other guys of his ilk (e.g. Ariza, Carroll, Dudley, etc), he will have stretches where he's scoring 18-20 regularly, and stretches where he's scoring 8-12 points. But he always comes to play and brings energy, toughness and leadership. When he's not scoring, he still makes hustle plays and keeps the ball moving.
He's the anti-Jeff Green.