The line...

This is the setup to that punchline:

At the approach of the birthday of the young son of the Klingon ambassador to Earth, the ambassador’s human friends decide to throw the little boy a party. They pull out all the stops, bringing in tons of food, presents and children’s entertainers. At the party, the humans notice that the Klingons appear very uncomfortable and are not taking part in the festivities.
“What’s wrong?” Asked one of the human attendees. “Don’t Klingons celebrate birthdays?”
The Klingon ambassador replies “Of course, but for a young Klingon’s birthday, all the partygoers dress as the most frightening creatures imaginable as a way to prepare the child to master his fears as he is becoming a warrior. If the attendees were dressed in happy and friendly costumes, it would indicate that the young Klingon was deemed to be a coward.”
Not wanting to be culturally insensitive, the human says “I’m very sorry, we didn’t realize. We’ll send the entertainment away immediately.”
Knowing that his friends did not intend any insult, the Klingon ambassador thinks for a moment and says “Well...the clown can stay, but the Ferengi in the gorilla suit has got to go.”