You can definitely argue this, and there are certain elements of the text that you can use to help prove this argument. Firstly, it is clear that she is one of the major characters simply because of the amount of time the text focuses on her. The story starts with her as she leaves her kitchen to go and accompany Mrs. Peters, and Martha Hale is also the person whose speech closes the story. This indicates the way that the author of this tale is using Martha as a prime character in order to communicate her message.
Also, let us remember that Martha Hale is the character who encourages Mrs. Peters to defy the male patriarchy of their world in concealing the one bit of evidence that conceivably links Minnie Wright to the murder of her husband. It is she who experiences massive regret as she sees the reality of the life that transformed the bright young Minnie Foster into an oppressed wife who suffered great loneliness and sadness. It is she who finally confronts Mrs. Peters with the reality of their situation and the problem that Minnie Wright faces:
There was a moment when they held each other in a steady, burning look in which there was no evasion or flinching. Then Martha Hale's eyes pointed the way to the basket in which was hidden the thing that would make certain the conviction of the other woman--that woman who was not there and yet who had been there with them all through that hour.
Martha Hale is therefore the most important character in the way that she is presented as the character the story begins and ends with, but also through her role in encouraging Mrs. Peters to defy the patriarchal authority of the world which so desperately wants to convict Minnie Wright for a crime that she committed.