Aside from the heroine seeing the MC as a black mass of power which wasn't in the WN at all, the others are jealous and arrogant because they grew up being considered geniuses and better than everyone else, so having this random girl appear shatters their view of reality and they mostly can't handle it, aside from the prince who is the only one that accepts there is no shame in not being perfect.
She also completely changes the story and various events never happen or get drastically changed, so the scenes that revolve around character development in the original game, where the main characters learn to acknowledge their flaws, become more humble, think things through or be nicer and more courageous don't happen, they are stuck as arrogant high school freshmen, if anything, they get worse than they were at the start due to jealousy and fear of the MC.
Except for the prince, he gets some other character development scenes and actually does go on to become a genuine ally of our MC, he even fights against the others and admits that while he still loves the heroine, he accepts that she is a worse person than he thought, sadly, like i said, the other three are stuck and get worse in the WN and this leads to deadly consequences for them.
The heroine in particular is stated by our MC to be much more incompetent and more of a coward than she was at the end of the game, due to having less experiences and events that made her change into an actually brave heroine.