Well yes the MC is selfish as she only wants to return to her world, and that’s why people love her. She’s realistic and determined. Other characters have sad backstories, doesn’t really excuse their actions though. Plus it’s not like it’s the MC’s responsibility to deal with their sad backstories.
Oops I meant to dislike
While i do agree with your initial statement about everyone turning out this way due to the world itself, Eris isn't the God to be taking care of them, in the situation she was, anyone would be looking out for themselves too, it was either be forced to be a wife to bunch of assholes who were too much of a coward to change their fate or leave to go back to her own world where while she had shit in her own life, she still tried to change.
Also you gotta remember that the world was forcing Eris to be stuck but she did everything she could to leave and she successfully did it, the 3 men didn't, the prince couldn't stand upto his father, the priest couldn't give himself the mental peace he needed and the warrior had a saviour complex which he could've worked on but he didn't. Eris didn't contribute to any of this so she doesn't deserve hate for this. She does have flaws but she isn't a villain here so we love her for that
All characters are miserable, they all became the way they are because of their sad backstory, all of them were forced to grow up to be this way, so Idk why everyone is loving the mc. I think she is selfish in only thinking about herself. But well… this story is about revenge and not losing your goal infront of temptation.