Of course he can be a good defender. He has already been a good defender like he was as a rookie. Will he be a good defender again? I keep hoping for it, but I think that ship has sailed. As his career has gone on, he keeps molding himself into this idealistic view of what a "superstar" is. He's completely abandoned effort on defense in favor of trying to do too much on offense. Spending less energy on offense might actually help him be more efficient because he would have to resort to less no pass possessions and have more spot up and system baskets.
He doesn't have the BBIQ or size to be a passively good defender. He has to do it with effort, but I just don't think he's willing to do that. It is true that many superstars conserve energy on defense, but that doesn't mean they become horrific defenders. They are still smart and positionally sound. They also ramp up their defense in the playoffs. Even a guy like Harden is tough to attack. Yeah he's resting guarding the worst player all the time, but if you do decide to attack him, it's not easy because he can guard inside and outside and he also creates turnovers with good hands. Same goes for most superstars.
Mitchell is also not remotely close to being an offensive superstar. If he was Damian Lillard good, I wouldn't care. But he's basically Collin Sexton good on offense (if that) while also giving 2% effort on defense.