When made human, Frank is shown to be the most handsome (or ugliest by monster standards) and this does actually connect the story of Frankenstein's monster. Victor Frankenstein made the monster in an experiment to create life, while described as beautiful, Victor Frankenstein ultimately rejected the monster after being disturbed by its eyes. Frank becoming incredibly handsome as a human makes sense because this was how Victor originally envisioned his creation.
When a stray beam from the Monsterfication device affects a zombie, he's overjoyed that he's alive once more... only to be distraught when he's bitten and subsequently turned back by a fellow zombie. This not only confirms that zombies used to be human, but they also remember being such, and it's not pleasant for at least some of them.
