Before attempting this question I had never even heard of the Mongolian Death Worm, so I decided to do this question so I could possibly learn about it. Here is some basic information which I was able to find simply by going on Wikipedia.
The Mongolian Death Worm is a cryptid purported to exist in the Gobi Desert. It is generally considered a cryptozoological creature; one whose sightings and reports are disputed or unconfirmed.It is described as a bright red worm with a wide body that is 0.6 to 1.5 meters (2 to 5 feet) long.In general, scientists reject the possibility that such mega-fauna cryptids exist, because of the improbably large numbers necessary to maintain a breeding population, and because climate and food supply issues make their survival in reported habitats unlikely.The Mongolian name is олгой-хорхой (olgoi-khorkhoi) where olgoi means large intestine and khorkhoi means worm, so full name means "intestine worm" because it is reported to look like the intestine of a cow. It is the subject of a number of extraordinary claims by Mongolian locals such as the ability of the worm to spew forth sulfuric acid that, on contact, will turn anything it touches yellow and corroded, which would kill a human, and its ability to kill at a distance by means of electric discharge.
From what I have read, I think that Gibson uses a Mongolian Death Worm in “Spook Country” in order to represent fear of an object which you do not know what it is, or fear of something which you are not even 100% certain exists. For Hollis, she is living in fear every hour of everyday because she has all of these people telling her what to do and what to believe, but they are all asking her to believe them based on good faith and without providing any hard proof, so she has no idea what to do or who to believe. She thinks that she should be afraid of Bigend because he might have been telling her lies the whole time, but it is that uncertainty which keeps bringing her back to Bigend because he could have been telling her the truth the whole time. All in all, Hollis’ life is filled with uncertainty and people telling her to do things and just expecting her to do it. It is almost as if Hollis is not in control of one thing in her life, a fact which she, and really any human being, hates.