I also have always liked the monster within idea. I like the zombies being us. Zombies are the blue-collar monsters.
The main jokes in this film are about big things, love and life and zombies - we all get that.
That's what we wanted to get across in that moment, particularly when Shaun goes to the shop when he's all hung over. He doesn't notice any of the zombies around him just because he never had before, so why should he at that point?
I'm obsessed with zombies. I like watching zombie movies and I read zombie books.