Context:

1. Scooby Doo episodes are usually about some dude who dresses up as a ghost or monster for some nefarious reason, e.g. scare people away from a theme park to put it out of business so they can buy the land for cheap or whatever. So episodes would usually end with the gang catching the ghost, unmasking him, and being him to explain why they did it.
2. Shaggy (the guy in the green t-shirt) is often asserted to be a "stoner" because he is paranoid and always hungry for snacks. Often also the rest of the gang, by association.

The Joke:

They have mistaken Patton Oswalt - who has a perfectly legitimate reason for being dressed as a ghost - for a villain. Because they are high.