Miss Maudie reacts surprisingly well. The expected emotion when your house burns down is likely sadness, depression, anger, and/or resignation. While her house is burning down, the goal is to save as much stuff as possible before it all burns, so there is not much time for reflective thought.
The next morning though Scout sees Ms. Maudie and is expecting her to be sad. Instead Ms. Maudie is quite cheerful. She tells Scout that she did not like her house that much anyway. She also says that she is looking forward to being able to build another house with a larger garden.
It's possible that she is putting on a "false face" in order to appear strong in front of the kids, but that's not likely. She admits that her number one concern during the fire was that it didn't spread to other houses. All in all Ms. Maudie is one positive lady.
The kids are expecting Miss Maudie to be sad, because well, her house burnt down after all! Most people would be sad in this situation, but before the fire Miss Maudie complained that she didn't even like her house much. After the fire, she admitted that she often thought of setting fire to the place herself if she wouldn't have been dragged of to jail for it. Miss Maudie is also excited to rebuild her garden and make it bigger and better, which is to be expected because Miss Maudie loves her flowers so much. Although, if you think of it Miss Maudie is getting older. Another reason why she might not have been sad is because she doesn't have all of her house to take care of anymore. Also, with old age she has probably learned that there are more important things in life than her house.