Jennings was expected to be an inefficient scorer as a rookie. It's no surprise that his efficiency has dropped. It will likely stay in this range (TS% 52%) or lower as the season progresses. As Jennings develops in future seasons he can try to fix that problem. I wouldn't be concerned at all by that now ...
I would be very excited by the solid defense + interest in improving his defense, the good rebounding, and the very good passing ability, the team first attitude and gym rat ways that Jennings has shown.