Clearly, Lemonade is about a black woman's quest for restoration. First and foremost, that is the theme.
Now included in that narrative is the story of how society has cheated the black female identity. It's just another arm of the black female experience, along with cheating, Lemonade is trying to share.
It's not supposed to mirror TPAB, which is more of a commentary of social injustice. Much like ASAT with songs like "Don't Touch My Hair".
TLDR; Pain and healing is what Lemonade is about. And Beyoncé happens to be black and so that happens to be the source of some of her pain. Which is addressed and personified by the grieving black mothers.