How would you describe the state of Mrs. Mallard´s mental health up until the time she heard the news of her husband´s death?
I would say that Mrs. Mallard's mental health was clearly in a very poor state before she heard of her husband's death.
The first thing that we are told about Mrs. Mallard is that she has heart trouble and that she therefore has to be told very carefully about her husband's death. This seems odd in a woman who is still young. I assume that her heart trouble is related to her mental state.
We are also told that she has lines on her face that "bespoke repression." (I am assuming that they were there from her life before she heard of her husband's death.) That implies that her life is tough enough to leave her with wrinkles on her face when she is still young. This, too, shows us that her mental state is probably pretty shaky.
However, we do know that she has a look of strength about her. Perhaps that is why she has been able to survive in a marriage that is clearly not fulfilling her needs.