What about the mutants? they must be surviving on SOMETHING. In order for them to survive, to get the appropriate minerals to live, etc. is by having some plants or fungi or something to eat. When the mutants die they are eaten, so to speak, by the plants, and so on. For the planet to be totaly dead, all the nutrients, minerals, and warmth would have had to have gone and any weather patterns would have had to have gone. Indeed, lavos would have had to have stopped the planet orbiting the sun and spinning. Unless lavos has giant gyroscopes and retrorockets on the surface, its pretty unachievable.
300 years is a long time to survive with nothing to live off by anyone's standards, even if they are mutants.