No, Hyenas are Scavengers. Decomposers are the one that breaks down organic matter and make it available to the producers and complete the food cycle.
Examples of decomposers are bacteria, fungi, termites.
Where as Hyenas are Scavengers, just like Vultures. Hyenas are a specialized type of carnivore, that feeds on dead animals.
It is a well-equipped scavenger and has a sharp sense of smell to find carcasses of animals. Also, it has sharp teeth that crunches the bones and its stomach has special acids to digest the hard bone.
That is the reason, hyena poop is bright white due to the calcium ingested through the bones.