Jotaro probably didn't train his ability because he didn't have to
Why train yourself when you won your battle and don't know any other person that could be your enemy
He just lived his life like a normal person, and although he seems violent, he is a peaceful and gentle person at heart
He also most probably suffered from PTSD. At just 17 he had to travel the world, risk his life watch his friends die and kill people, which isn't an easy thing to do. And it seems like Jotaro's first encounter with Kira proves his PTSD as the fight resembled the Stardust Crusaders' fight with Dio