It's hard to tell exactly what the play call and scheme was, but it looks like a cover 3 (Maxwell, Thomas and Sherman are responsible for each deep third of the field) where Kam was covering the flat on the side that Jackson was on.
If that is true, Kam's responsibility was to cover any receiver in the flat and stick with them. What was brutal is that Jackson is a burner and Kam just got beat deep on basically a wheel route. The play can be chalked up to great execution and decision making by Cousins to get the ball where it needed to go.
For what it's worth, that play call was great against a cover 3. The skins had two receivers attacking the seams which opened up space both under and on the outside considering Sherm, Thomas and Maxwell (Maxwell stays with the outside receiver on this play) at that point have to choose who to cover based off where the QB is looking and where the open space is.
IMO, no one really had any bad execution on that play because the call against a cover 3 was pretty much perfect and one of the best ways to attack a cover 3.