Not sure where you're getting the ORTG and DRTG numbers from. Mitchell's offensive rating is 103.4 and his defensive rating is 102.8. Even going by nba.com's less accurate measurements that use an estimated # of possessions instead of an exact amount, his offensive rating is 101.1 and his defensive rating is 101.6.
Mitchell's TS% is suffering because of an abysmal first few games. But in the month of November, it's at 52.1%, which is actually decent for a rookie shooting as much as he does.
NBA Reference has him at a 92 offensive rating.