What are the guidelines for a decent shooter?
If decent is average or slightly above, Rondo can eventually become a decent shooter... When you've taken thousands of reps using a specific technique, it takes thousands of reps to change it especially if your previous technique continue to rear its head).
Rondo allowed the criticism to alter his game, instead of tailoring his training to enhancing his exceptional skill of getting into the paint by obtaining a signature high % shot (ala Tony Parker, his tear drop, layup, and ability to draw foul calls) , he chose to improve his skill in one of the lowest % shots in basketball (the mid range and 3 point shot).
Now, he's shooting at the worst FG% of his career (and has seemingly regressed at one of his former strengths) because he's trying to satisfy cornballs criticism by proving that he can shoot.