This is why you cannot hide your heat signature like Schwarzenegger in Predator
It may seem like the idea of a predator using heat detection to locate prey may seen farfetched to the uninitiated. However, reptiles like snakes can do the same in the real world. Of course, the snakes do not have infrared heat detection in their vision. They rather have openings in their eyes called pit organs, which are coated with a layer able to sense infrared radiation from warm bodies as heat and not as light waves. Due to the lack of real-world Predators but abundance of Schwarzenegger, we are fortunate the creatures do not exist or many of us would have been hunted easily for using mud as concealment.
IMDB describes Predator as 'A team of commandos on a mission in a Central American jungle find themselves hunted by an extraterrestrial warrior.' We will assume everyone has seen the 1987 Classic film but light spoiler alert ahead if you have not seen the movie yet.
Mud has long played a part in ocular disguise for those wishing to conceal in the environment, but the advantage is just visual, not thermal. In the film, however, Dutch (Arnold Schwarzenegger) adapted and overcame by using a technique adopted by pigs that utilize mud baths to cool off. Pigs lack sweat glands to keep them cool, but mud presents a similar effect since the water in it cools their body. Everyone expects mud on the field op but not like this.
The problem with improvising camouflage with mud is that so many things move against you. To begin with, whether you spread it sparingly or firmly, it will only minimally drop your body's warmth, but not below that of your surroundings. Anyone who has undergone jungle warfare training can attest to how hot an humid those conditions actually are. The mud will naturally heat up even the thickest caked on mud - and you'll still be wet. When it dries, it will further give you away as it dries. Unfortunately, the heat signatures will still get detected, and the predator will locate you and you're going to sit in his trophy case.
This is particularly true because the water will vanish rapidly from regions where the mud is less thick, warming back to core temperature speedily. For the infrared camouflage to work, you will need something that can keep your temperature below that of your surroundings. The body has some warmer areas than others, and some areas are very hard to conceal without special tools. Unless you are fortunate, it might be an issue because infrared will make hot air from your mouth or nostrils very plain. Holding your breath and closing your mouth is not an option either because you will eventually need to breathe.