Did it hurt? Enes Kanter got a max contract. Dante Exum suffered, but that is more a function of tearing his ACL in his 2nd year. He had a relatively successful rookie year being a starter.
All signs point to sitting out a year, as long as your a top rated prospect, actually raises your stock.
Mitchell is measured as a true 7' with a 7'4 wingspan and top level athletic ability. I dont think him sitting out a year takes him out of the lottery, and best case, it actually raises his stock if some of the other top big man prospects struggle or get injured.
If I had to bet between sitting out or playing for a mid-major.. I'd sit out (only speaking draft stock).