Scout and Jem have mixed feelings about Christmas for exactly the same reason that many kids/people have mixed feelings about Christmas (including myself). Scout and Jem love the holiday. That "Christmas spirit" is working within them. Presents are exciting, food is good, the tree is pretty, etc. Scout and Jem like all of that.
Unfortunately, that is about it regarding the Christmas in the book. Scout and Jem must spend Christmas at Finch's Landing with their Aunt Alexandra and Uncle Jimmy. They are not excited about that.
"No amount of sighing could induce Atticus to let us spend Christmas day at home."
Scout doesn't particularly like spending time with anybody from that family. Aunt Alexandra disapproves of the way Scout behaves; Scout is too much of a tomboy in her opinion. Scout should be acting much more like a lady. Uncle Jimmy isn't disapproving, he just doesn't talk . . . to anybody.
For Scout, the worst part of Christmas at Finch's Landing is having to spend time with her cousin Francis. He's a brat and a tattle-tale, and he couldn't be more of a polar opposite from Scout if he tried. His favorite Christmas presents are things like a book bag, bow ties, and shirts—all pointless in Scout's opinion. Scout summarizes Francis like this:
"Talking to Francis gave me the sensation of setting slowly to the bottom of the ocean. He was the most boring child I ever met."
Scout and Jem love the holiday, just not the family they have to spend it with. I'm sure many people have felt that before.