What did Mercedes do that made the trip even more difficult for the dogs?

Mercedes made the trip more difficult for the dogs by overloading the sled with unnecessary supplies and items, which increased the weight and made it harder for the dogs to pull. Additionally, her emotional state and lack of experience in handling the dogs added to the challenges, as she often panicked and failed to manage the team effectively. This ultimately strained the dogs and hindered their ability to perform optimally in harsh conditions.