I'll be honest, I had to look up 'A Christmas Karen' because I didn't know the book, but it seems to be a self-published holiday romance or comedy. From the Goodreads page I found, reviews are pretty polarized. A lot of readers think the plot is a fun, modern twist—taking the classic 'Scrooge' archetype and making her a demanding, entitled customer in a festive setting. They say the holiday setting, probably a small town with decorations and a grumpy elf love interest, is cozy and does the job.
But the one-star reviews are brutal, calling the plot thin and the main character irredeemable, not just grumpy but genuinely unlikeable. The setting gets called generic 'Hallmark movie but mean.' I guess whether you like it depends entirely on your tolerance for a protagonist who starts out truly awful. I'd probably read a sample first; it seems like a love-it-or-hate-it kind of thing.