I didn't understand it that way. I understood like she died due to a combination of barely eating, mental instability and not really doing anything during the day. There are people who die because they decide they don't want to live anymore and their body shuts down.
That's what it said at first, that she lost her will to live because her workaholic husband was coming home so late and so often that he believed he was cheating on her and it broke her heart. Then it was saying that she died of lack of nutrition, not because she didn't feel like eating at all, but because she doesn't like vegetables to the point where they have to now hide it in her food like one would do for a child. Like what the heck
She says that maybe her mother's weak body is partly due to her lifestyle choices. So she's trying to get her eat healthy so she gets more energy and spends less time in bed. http://www.mangago.zone/read-manga/i_won_t_become_a_villainess_i_m_just_a_normal_duke_s_daughter/nbt/mpn_chapter-2196124/19/
It's not mentioned so far but that might again change the way she thinks since she's not lying alone in a bed most of the day thinking pessimistic thoughts.
What kind of mother does because she doesn't get enough nutrition because she doesn't like eating vegetables? Lol what the heck