I think he can be a good situational defender against 3's. I don't think his feet are that quick on the defensive end, but his length obviously helps.
I actually see him as the opposite of Okoro. Okoro is very good at positioning and anticipation, he guards on-ball about as well as he can, but doesn't have the length to contest all shots.
Guys will be able to get a lot of separation from Mobley when he's on an island but with his length he can recover and contest (both at basket and on perimeter).
I also think you will all be very disappointed with his handle. That should have been obvious in summer league as it was in college.
Just because you "can" doesn't mean you "should". Every big in the NBA has handles that would probably surprise a lot of people. Just look at Andre Drummond. In a vacuum he "could" but as we all saw he definitely "shouldnt've".
Too high, too loose, too predictable. Once guys see the tape and put a body on him he's toast.