Question

is decaying matter always warm?

Answer 1

Yes, the decaying matter is always warm. Decaying is the decomposition reaction which is exothermic (that is heat is released during this reaction).

since you are just 7th standard the answer given below would be more appropriate
The process of organic decay releases carbon dioxide, water, plant nutrients, synthesized organic carbon compounds and energy. This energy gets trapped inside the layers above and hence the decaying substance stays warm.