One of Lindsay Clancy's supporters says the Massachusetts mom shouldn't be on trial for murdering her 3 children ... she says the mental healthcare system failed Lindsay.

Renee Kimball joined us on "TMZ Live" Friday ... and we asked her what justice looks like for her.

She says Lindsay deserves to get the help she begged for and to be able to move on with her life and help others who are going through postpartum psychosis.

Renee organized Thursday's rally at the Massachusetts courthouse where Lindsay is on trial, and tells us her group's support for Lindsay is not meant to justify the deaths of her children.

Instead, she says they hope to raise awareness about postpartum mental health struggles and push for systemic change.

Renee tells us the trial has encouraged women to openly share experiences they had previously suffered through in silence.

If it were up to her, Lindsay wouldn't even be on trial ... and she explains why she believes the healthcare system should be tried instead.