Well, knowing that Family Guy likes to pick on certain minorities, it's sorta like--well, that's what they're doing. Ever heard a rabbi speak, on TV or in real life? They kinda have their own dialect. I'm sure you'll hear some of it in tonight's episode.
The real meaning for goy is "nation". Goyim is the plural form which means "nations". However the word is most commonly used now non formally to mean a non Jewish person.
I enjoyed the episode, better than most of the episodes in the past. Not up to Road to Universe but still good. I was not expecting Stewie to do that to MegXD