I think while the scene may feel dramatic to some folks it isn't an exaggeration to see her mother respond like this when the entire comic has been littered with hints at just how serious the abuse is, while the art style and main character herself downplays it. Like we didn't necessarily know she was very clearly malnourished and was being denied new clothes and necessities until the ml's narration and her party where she wore the same formal dress.
But we did see how her mother emotionally abuses her and manipulates her, we did see that she would take the place of a punching bag for her mothers often random violent physical outbursts for the sake of the servants who even looked down on her, the butler could demand she do things which is so wrong. She even was controlling her appearance, likely thru withholding those necessities and dictating what hair, makeup, clothes she could have to prevent any man from wanting to marry mc, ever. Probably also related to her denigrating mc's appearance, intelligence and skills constantly so she never has the confidence to see leaving as an option on her own (which works in the og timeline).
And then there's her mothers seeming resentment of men and of the idea of her daughter leaving her. She pretty clearly doesn't want that bc she loves her, instead it seems to be a control thing. She needs her in view to keep being a tool for her and for her son, bc that's the only reason she exists in her mothers eyes. She isn't a human, or her adorable baby, only the shitty older son is her baby.
I mean the entire premise is based around that she should dirty and chip away at herself so her dumbass incompetent psychopath brother can have the throne. (he seems to have inherited mum's resentment complexes too) This kind of baldfaced violence from a woman who has gotten away with this level of abuse for so long? Not so surprising imo.
Her daughter is doing things without her permission, not necessarily for her son, or for her mother, she's telling people off who are her mother's people, she's throwing a party without her mother's permission, with people who are not important to her mother or brother, for her own benefit. The awful woman must feel like her daughter is wretching control from her.
And then mc needs someone to be on her side bc in the original timeline she really did have no one to help her improve her life bc everyone immediately around her either had no power to help her (the kinder servants) or were actively using her and couldn't give a toss about her unless it's to tie up a loose end. I dread to think what would have happened to her if she didn't use the ml as a sort of shield for some moments in this story.
Her mother still doesn't get that that her behaviour towards her daughter isn't normal and will be heard by the people in nobility for the vile treatment it is and it may well even result in a severe punishment as she is now the official fiancee of a member of royalty. She played herself.