The narrative exists because stats back that up over his career. He does disappear at the end of games sometimes, but that is his teammates and possibly coaching fault not his. If no one passes him the ball or the coach doesnt draw up anything for him then he cant do much. When given the chance he is solid in those situations. Having a bad shooting game or missing sometimes in the Clutch happens to ever player. Even the greatest clutch players of all time had that a lot.
Mitchell was pretty clutch the last couple years in the last few minutes of games but he has struggled this year. I think giving him more opportunities to work on it in the regular season is a good decision though. But I feel like he is not learning from his mistakes this year well. He needs to learn to pass when he is doubled and not settle for bad 3 point shots. He also needs to use picks and not go iso as much when it isnt working.