Queen Margaret has a small role in the play, but she is important in representing all the powerless people who have been hurt by Richard III and in general by the bloodshed and disruption caused by the War of the Roses. She has no real say in what happens and is dependent on those who killed her relatives and put her in this powerless position, but she does have a voice—and she uses it.
Her curses in act 1 are famous, both for being so venomous and also for coming true. If Margaret is the stand-in or proxy for all the powerless people in England who have been wronged by abusers they can't fight back against, Shakespeare seems to be saying that God is listening to their complaints and poetic justice will be done.
Margaret is a one-dimensional character, defined by her bitter and relentless anger, but we also sympathize with her. She has every right to be consumed with rage. As she says to Richard III:
Out, devil! I remember them too well:
Thou slewest my husband Henry in the Tower,
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