It takes a special degree of skill to write a parody like Pride and Prejudice and Zombies and still hold true to the essence and feel of the original. I think that was accomplished. I suppose it's cheating if your goal is "original literature". If your goal is "make a classic more appealing to the newer generations" or "entertain" or, honestly, just to write a funny version of a classic...I don't think it's cheating.

You have to define the game your playing before you can call "Cheating"