Mordecai is dealing with the flu. He'd wouldn't admit it, but it clearly takes a toll on his demeanor and shrivels him into an exhausted, sick and feverish grumpy man with a scowl more snappish.

He dealt with a high fever and nausea, alongside body aches and headache.

He hasn't slept well enough. It shows by the bags underneath his eyes and it's a bother.