One of thew actors says the whole thing tastes like chocolate and chemicals, which would seem to indicate that’s more in the mixture than just coffee.
If nothing else the texture of that seems awful, and the idea of playing a zombie in the movie and having that in your mouth at all times seems far from pleasant.
Using chocolate sauce for blood has been a classic technique that goes back to the days of black and white film and while chocolate and coffee are two great flavors that belong together, there’s something really wrong with the idea of chocolate sauce mixed with instant coffee crystals to create the “mouth goo” for Zombieland: Double Tap.