When one spectates a mob in Spectator mode, it displays the screen from their perspective.  Sometimes it will even provide a screen-rendering effect for particular mobs, such as a green tint for creepers.

However, spectating a mob with a pumpkin on its head does not show the overlay.

How to Reproduce:

Execute the following command: </summon zombie ~ ~ ~ {CanPickUpLoot:True}> (or just build a snowman).

Give the zombie a carved pumpkin and wait for it to pick it up.

Go into Spectator Mode.

Right-click on the zombie.

No pumpkin overlay is shown.