She's simply the voice of despair in the play. We meet her just after her son is murdered, which she first responds to by saying, "What world of grief […] Oh, where's the author of this endless woe" (2.5.38-39).
And she's dead on, because endless woe perfectly describes her role. Like Hieronimo, she passionately cries out for redress throughout the play. But unlike Hieronimo, Isabella never gets any kind of satisfaction. As a symbol for women of her time, her complaints and despair never find a public voice, leaving her to express her sadness and rage to only herself and her maid.
Poor Isabella is stuck at home throughout the play. So her expressions of rage and torment never leave her bedroom or garden. Women in the 16th century would totally relate to this kind of captivity (to be fair, some women in the 21st might, too). And while Hieronimo is equally unhinged by the death of Horatio, at least he gets to vent his depression and rage while he pursues some form of public justice.
Isabella, on the other hand, is left to smolder until her emotional heat completely consumes her. Just before taking her own life Isabella stages her own form of revenge by cutting down the garden in which her son was brutally murdered. After destroying the garden, she curses it to never bear fruit again while also cursing her own reproductive powers: "And as I curse this tree from further fruit,/ So shall my womb be cursed for his sake." It's a horribly sad scene because we know she is an innocently hapless victim in a world of male violence.
Her misplaced vengeance and self-sacrifice stands as a tragic symbol for female victimization. In short, she's a pure victim who destroys herself as a result of the sins of others.