Never got around to watching this one, but interesting they got that out of such a silly tune I listened to 40 years ago.
I watched it all the time as a kid, but as an adult, it's one of my favorite so bad it's good movies. It's honestly amazing just how bizarre this special is.
They try to adapt parts of the song to be more kid-friendly, but it only overcomplicates things. For example, the verse about Grandma (yes, she never gets an actual name) drinking eggnog and forgetting her medication is recontextualized into her having an egg alergy she needs the medication for. Also, not only does Grandma survive getting hit by Santa (who took her to the North Pole for medical care), it's revealed to be a complete accident because her fruitcake was spiked with reindeer nip that drove the reindeer towards her.
However, at the same time, there's a lot of legal talk that has no place in a cartoon for kids. The main plot revolves around Cousin Mel (who was only mentioned a single time in the song and is now the main villain) wanting a rich CEO to buy out a general store Grandma owns. After Grandma gets hit by the reindeer, she's presumed dead, but Mel can't sell the store because Grandpa has power of attorney, which she has to trick him into signing away after he sings about Elvis. When Santa explains what happened Mel and the CEO, Mel sets up a plan to kidnap the fully recovered but now amnesiac Grandma and sue Santa for her disappearance in the first place. The special has serious courtroom terms during the trial of Santa Claus for some reason.
And then there are other weird elements, like the main character's last name being Spankenheimer, a police code for disputes over the existence of Santa, Santa's email address being public knowledge, and an actual line having a character saying "Better get looking for the old broad."
It's my favorite kind of bad movie: absolutely stupid, but absolutely sincere in its stupidity.