Madame Leota was well aware it would be Christmas Day in five days. Just because she was in a glass ball in a very dark ride, did not mean she wasn't aware of the date.
She also was aware of the West Coast Haunted Mansion and how they had a Nightmare Before Christmas holiday overlay. She wasn't sure whether she'd want that for the East Coast Haunted Mansion she occupied, but it would be nice to try it.
With the lone exception of the singing busts belting out Ding Dong Merrily On High on a loop after Park closing, there was nothing in the attraction to suggest "Holidays".
But maybe, she reasoned, it didn't matter. As long as they were all together, singing Grim Grinning Ghosts to thousands upon thousands of guests while they waited for the Christmas parade in uncomfortable weather because nowhere else was open, not having Christmas decorations or an overlay wasn't the most important thing in the world.
Laughing at tourists having to wait an hour in line for the restrooms was. And although Madame Leota didn't see it for herself, she never had left the attraction after all, she got to hear all about it.
Yes, lines for the restrooms get crazy high in Disney World at Christmas. One time, I went on the 4th July, and had to wait half an hour to go to the restroom. I believe Christmas and New Year's Days are worse than that.