Mattie isn't truly troubled by this. She may be worried that they are returning when the fever has not totally passed and the plague might return.
The return is a sign that the plague has passed for the most part. Mattie's coffee house will thrive with the returning customers. With more people returning, the town can be restored to normalcy with trade returning and clean up and restoration beginning. It will also reunite her with her now ailing mother. Mattie can show her mother her new found maturity and gain the independence of a young adult. Those who had kept to their houses to avoid the illness can now come out and reunite with loved ones. This will lead to Mattie and Nathaniel rekindling their friendship and budding romance.