So, now that I've played this game a crap ton, I still don't see any reason for Ganon to be called Calamity Ganon. It feels really awkward and corny? It doesn't add anything to him as a threat or concept. It's kind of redundant, really. He's always been a calamity. That's his whole point in the timeline and narrative.
Yeah just The Calamity would've been better.
Calamity Ganon makes it sound like somebody (or Ganon himself) specifically named him that way and everyone somehow understands it, but as far as I know he's just a big evil thing no one should know much about.