Her head is gone. If that is Big Mom it means she is already dead.
The one surrounding her body is not her clothes. What is then? It doesn't follow.
Some argue that Kaido's coat melted (no magma-proof), but in the silhouette Big Mom's kimono is still in her body for ±30 minutes submerged in volcano. It doesn't follow the logic. But we can't know for sure, of course.