Nothing in D&D is mythical, even the gods. D&D takes content from myths, legends, and 20th century fiction, views it only in terms of its powers and capabilities, and stats it up thereby rendering it meaningless.
I gotta love when folks go to absolute extremes, like "meaningless".
In and of itself, a dragon of myth was meaningless - it is how the fictional element is used, and in what context of story, in which humans find meaning. Meaning is found in the story overall, not in single parts.
If you kill Tiamat in D&D can you make the world out of her body?
Well, kind of hard to create the world when the world already exists, so, I wouldn't expect to.
Hm. Beowulf didn't build a planet out of his dragon's body. He's just a guy who kills a few monsters. So... is his story meaningless?
To you, are, "myths," limited to, "godlike beings that exist before the world"?