The main thing that Mercedes does that hurts the dogs, even though she is trying to help them, is to give them extra food. She gives them extra food because she feels sorry for them. As we are told in the book:
And to cap it all, when Mercedes, with tears in her pretty eyes and a quaver in her throat, could not cajole him into giving the dogs still more, she stole from the fish-sacks and fed them slyly.
But there are two reasons why Mercedes’ actions really do not help the dogs at all. First, we are told that overfeeding them early in the trip will eventually be bad because the food will run out. If the dogs eat too much early, there will be nothing for them to eat later. This is, indeed, what happens. Second, we are told that what the dogs really needed was rest, not more food. Therefore, Mercedes is really not helping them.
What this tells us about her, Hal, and Charles is that they are not good planners or organizers. They are, instead, careless and impulsive. This will end up getting them in very grave trouble later in the book.