Looking up a sample of rookie guards from the past 6 or so years whose efficiency dramatically improved after their rookie seasons:
Rookie De'Aaron Fox was insanely less efficient as a rookie than Keyonte, and scored much less per 36 as well.
Rookie Trae Young's efficiency is identical to rookie Keyonte's (albeit on much higher volume)
Rookie Anthony Edwards' efficiency was a lot worse (similar to Trae, higher volume).
Rookie LaMelo Ball's is identical.
Rookie SGA's efficiency was very meh (better than Keyonte's for sure, on lower volume), but my God has it exploded upwards since then.
None of this is meant to compare Keyonte to these players. More to underscore the point that rookies, even ones that develop into great players, rarely have good efficiency. It's certainly not a reason to be a doomer about their future.