The first Christmas card was born of outdated etiquetteSource: Pinterest
Christmas cards came about in a confluence of yuletide cheer
Victorian Christmas cards were bonkersSource: Pinterest
Rising literacy rates helped popularize the cardsSource: Pinterest
Mass production in the 1860s created millions of cardsSource: Pinterest
Many designs were based on folkloreSource: Pinterest
I believe the cultural interest in fairies, secret places, and strange creatures that developed, maybe beginning with seances, elves, and so on, in the Victorian era may have something to do with some of the fantastical Christmas cards.
Victorians were more festive, not morbidSource: Lilly Library, Indiana University Bloomington
They're only odd to our eyes. The Victorians had a different idea to what Christmas was about---not particularly Christian, but a time of good humor. You may find a mouse riding a lobster strange---I find it funny. It's horses for courses.