Maybe the grinch is not that well-known, as for grinchy to be translated by making up the respective adjective (grinchisch, grinchmäßig, or whatever the correct term is).
Is there a German word, a (preferably a funny way, as in order to stress the joke character) way of putting together
nicht weihnachtsbegeisterter Mensch
Weihnachtsverderber (don't know whether it's correct, but seems aggressive) ?