While Rotom's Ghost typing makes it strong against many common Pokémon in a Ghost-only playthrough, the Electric typing would prevent it from being considered a "pure" Ghost Pokémon for the purposes of this type of playthrough.
You could still use Rotom in your playthrough as a powerful ally, but you'd have to be strategic about how you use its Electric moves, as they wouldn't be effective against the Ghost-type Pokémon you're trying to catch.