Probably not, but just because someone else does wrong, that justify you (or in this case Christie) doing wrong?
I’d be curious to know why it would be wrong for Christie to appoint a Republican. He’s entitled to appoint whomever he chooses, and from what I’ve read of state statute as posted elsewhere, he is under no legal requirement to choose a replacement from the same political party as the deceased.
It’s his choice. If the voters disagree with that choice, they have appropriate recourse through the ballot box.
He can pick whomever he wants. Fairness really isn’t an issue either way.