I didn't read the IDW comics (however I really want to at some point), but my guess is that mirrored, evil versions of Celestia and Luna have different personalities from their regular counterparts too. As suggested by the word "mirrored", the exact opposite kind of personality, in fact. Yet, `evil celestia` and `evil luna` imply regular Celestia and Luna.
And when it comes to designs - I tend to disagree too, designs of Nightmare Moon and Daybreaker are not THAT far off from Luna and Celestia in their usual form.