I disagree with you. He was a fluid athlete for a SF with a good explosiveness. If you think he's stiff, just watch a few highlights.
This video has highlights from his 2017 McDonalds' All American, 2017 Nike Hoop Summit, Missouri scrimmage, and Missouri preseason games all combined. Look at 1:24, 1:57(shows his explosion), 2:02(fast break dunk), 2:36(alley-oop), 3:07(1foot dunk).
Of course there's some questions whether or not he'll be able to regain the same athleticism after his back surgery. If you watched him in the 2 games at Missouri, you don't see the same type of explosion, but he said he was only playing at 65%. You could also tell his conditioning wasn't there yet.
His ball handling is a weakness right now imo. Not weak in terms of having 0 handles, but weak because he has lack of advanced dribbles. He does a great job using both head and ball fakes, but when they don't work, he's SOL. In ISO situations, he's limited because he doesn't have the ball handling to get him all the way to the rim, nor does he have the quickness to "cheat" himself there. You often see him rely on his strength which doesn't workout well. However, don't mistake his lack of ball handling for his ability to create offense. He actually does a good job creating space solely by using his head fakes. Defenders still have to respect potential drives to the basket, so they give him a fair amount of space. Another thing that "compensates" for his lack of driving ability in the half-court is his very good shooting ability. Being 6'10, he can easily shoot over the top of defenses. He can be trigger happy, but at least he has a quick trigger.
2:13, quickly shoots off the curl, although he misses it very short. had a driving lane if he wanted to take it.
2:23, 15ft curl, as you see in this scenario(like the last) he has the option to shoot or drive. he puts 1 dribble on the floor to get the defender to back up just enough so he can get his shot off.
2:40, quick catch turnaround J after he realizes defender is far enough off him
2:55, catch and shoot 3pter on PnP
3:53, slips back for a 3pter
These are just some examples of his quick trigger. He missed most of them, but you can see he has the right idea with those shots. Anytime a defender gives him space, he'll have no problem letting it fly.
My bigger concern with MPJ is his injury and his lack of a mid-range game. I know it's dinosaur talk, but I want to see him have 3 level scoring. Oddly enough, he's better with his back to the basket down at the low-post than he is in the mid-range.