Yeah a dress jacket will "bunch". What will NOT "bunch" is a Dress Shirt ANCHORED by a Dress Tie behind the dress jacket. The bullet hole in JFK's dress jacket lines up with the bullet hole in JFK's Dress Shirt. JFK was POTUS and before that Rich. His dress clothing was Fitted. He was Not running around in rags off the rack at Robert Hall's.
Why not date this pic? POTUS flying Commercial? HAHAHAHAHAHA
With the right arm elevated, there's usually a jacket bunch at the back above the right side of the midline. The top of the shirt collar should be above the top of the jacket collar at the nape, but on the right side it becomes obscured by the jacket.
In the Zapruder film, the right side of the top of the President's jacket is both higher than the left side and is substantially higher than the level of the necktie knot.