That's a tough question. We know that by choosing Jack, Ralph gives Jack a power base that Jack later uses to split and then take over the tribe. But you have to ask yourself what would have happened if Ralph had denied him that official authority.
We know that Jack is concerned about power from the beginning. As the first of my linked pages points out, his preference for being called Merridew instead of a "kid name" shows his arrogance. He believes he should be leader. If Ralph had denied him any official title, he might have made a play for power even earlier. The question is whether this play for power would have worked.
Much of Jack's power came from fear, and the fear was generated by the "monster" on the mountain. So, one argument might be that Ralph did make a mistake. If he had forced Jack to "fight" for power, the boys might have had a chance to see what an arrogant and selfish child Jack truly was. Without fear of the "monster," their judgment might have been better.
However, another argument says that Jack might have been successful in taking power if Ralph hadn't shared his authority. He was taller, was the "chapter chorister and head boy", and clearly had a stronger desire to be leader. If he had usurped Ralph earlier, Ralph might not have grown into the good leader we see at the end. By having boys look up to him, Ralph developed into a good person, and if Jack had ripped power from him, that might not have happened.