The creature is probably alive
Keep in mind that the Bag of Holding is resistant to fire damage:
a magic item is at least as durable as a regular item of its kind. Most magic items, other than potions and scrolls, have resistance to all damage.
However it's hardly fireproof, since nothing in the rules says it can't be burned.
If the bag is destroyed, its contents are "scattered in the Astral Plane" (and probably lost):
If the bag is overloaded, pierced, or torn, it ruptures and is destroyed, and its contents are scattered in the Astral Plane.
But the important thing is, breathing creatures suffocate inside the bag:
Breathing creatures inside the bag can survive up to a number of minutes equal to 10 divided by the number of creatures (minimum 1 minute), after which time they begin to suffocate.
So if the creature survived inside the bag, it is now somewhere in the Astral Plane.