I don't think they were making fun of the fake interpreter or any ASL interpreters or anything to do with ASL.
What they were making fun of was that this whole screwup happened at all. Somebody did not do their job checking this guy out to make sure he really was who he claimed to be and had the capability he claimed to have, when he was going to be within a couple of feet of President Obama and other world leaders.
I meant," said Ipslore bitterly, "what is there in this world that truly makes living worthwhile?"
Death thought about it.
"Cats," he said eventually. "Cats are nice."
-- Terry Pratchett, Sourcery