I don't think Mrs. Baker is necessarily "anti-holiday;" it is just that this year, the holidays, to her, serve as such a strong reminder that her husband is away fighting a war. The holidays are always a difficult time for those who are unable to be with their loved ones, and the stress created is especially intense for people who know their loved ones are in danger. Mr. Baker had been deployed to Vietnam only three months earlier, and news about the extremely critical and dangerous conditions for soldiers there are shown with vivid description nightly on the news. To make things worse, Mrs. Baker's co-worker, cafeteria lady Mrs. Bigio, has just recently learned that her husband, who has also been serving in Vietnam, has been killed. Mrs. Baker is missing her own husband acutely, and the memories of happy holidays they have spent together only makes her longing for him more intense; the fact that she is constantly aware that he is in great danger and might never come back only adds to the stress she already feels. It is no wonder that Mrs. Baker is not in the holiday spirit.
Holling, who is still predominantly self-absorbed in his orientation towards the things going on around him, does not think about the reasons for Mrs. Baker's apparent aversion towards the holidays. He only observes that she doesn't put up any decorations, and that she is "not in a happy holiday spirit." Later, as he grows in maturity, he develops the ability to perceive causes for the actions of others and can empathize, but for now, he is so absorbed in his own concerns that it does not occur to him that Mrs. Baker is not able to celebrate the joy of the season because of the pain of her husband's service in Vietnam (Chapter 4 - "December").
Thank You very much dymatsuoka. Your answer helped me a lot in understanding Mrs. Baker a little more.