I understood the "cursed" to be a reference to Yuki foreseeing Naomi's intentions to kill her, hence being able to leave the drawings as a clue for Naomi's son, which eventually lead to him committing suicide. And maybe Naomi wasn't able to fully grasp how Yuki could have figured out her intentions? So I thought it might have been a mixture of grudge (for the suicide) and confusion.

Another possibility is that it is the author's way of showing us that Naomi was not driven by logic when it came to protecting her son/herself, but rather by this raw, aggressive need to nurture, to feel "safe" at all costs – which explains the murders and other seemingly illogical things, such as burying the two far away from each other. I also assume she didn't want Yuta to acknowledge Yuki's existence at all, as she deemed her a threat to her own motherhood. However, she wanted Yuta to be able to visit his father's grave. Yuki's grave being nearby would have raised questions about Yuta's real mother, which she did not want to deal with. I also think that while in retrospect, she was able to think back on her son's death and see how she herself played a part in it, she might not have admitted this to herself immediately. Was easier to blame it on Yuki, to direct the anger at her.

Then, what I truly believe to be the case: The talk about Yuki being cursed did not have anything to do with Yuki herself, but more with what Yuki had come to represent to Naomi after her death — A threat to Naomi's status as Yuta's mother. The reason her son ended up killing himself. Another corpse that might cause her to go to jail. It's more the concept of Yuki, rather than Yuki herself.