That was definitely something I thought. Everyone gets mad because Ariadna isn't as mature as the others. But I think this is normal and nice. From the moment I read it, Ariadna was a baby who remembered only her past life for me. And if Ariadna had been crying like you said, she probably wouldn't exist. This was an important part of the father and daughter's relationship. Catiel was a person who did not like and liked. He had a structure that survived on her own and did not trust others. For someone like him, Ariadna's existence is really necessary.
Ariadna is a character that I really like even though she has lost her sweetness a little. She thinks of everything around her and considers all this as her family. If Ariadna had been as mature as in her old life, the connection with Catiel would be really weak.
That's why I think Ariadna's current characteristic is the best, no matter what others say, I have no trouble with Ariadna's new personality.
I just want to remind you this is REINCARNATION STORY, not TIME TRAVELLING STORY. It's true she has a memory of her past life, but she was born in new body in another world. What more can she do in her child body except act like one, sleeping, eating and playing. She died at age 26, the same age with Caitel when Ariadna was born, so if you want her to fall with someone in her mental age, it would be her dads friend, don't you think it's more gross? No matter how old is her mental, her body still that of kid and she said so herself that she feels like her mental age regressing back to that of a child because the people around her treat her like a child and she was expected to behave like one. and why the author bother to make her remember her past life, I think it was for the sake of her survival, if she is just a new borne baby with no past memories, Caitel would already killed her by now. with her knowledge, as a new borne, she knew if she acted like real crybaby, he would kill her and as a child if she doesn't act cute, Caitel would kill her.
The difference between this manhwa with similar manhwa like 'They Say I was born Princess' was the fact how Ariadna grow up into different person in different world (her face change) whereas Sanghee grow up to be just herself but in different world (her face remain the same). When Ariadna still a baby, she sound like her old self, but as time goes by she adapted to her new life, she begin to think and accept the fact that no matter how cruel Caitel is, he is indeed her own father who share the same blood and thus long for him. at first she only think of how to survive and tries to win Caitel affection but now she is sincerely love him though she still doubt and fear him sometimes, but we can see the real bond between father and daughter those two shared. and when she sees other kids with their mom, she suddenly miss her mom, not her mom back in Korea but her new mom who died when she give birth to her in this new world. But in that another manhwa, Sanghee never thought of herself as a new person, she is just someone who stuck in another world and do everything to survive not to mention her past lover was there to. and she don't even think of her mother as her own mother, more like someone who take good care of her, and so does with her father and brother, she never think of them as family (maybe it will change later on)