It could be that Team 7 is just being ignorant. They might be so focused on their own powers and goals that they overlook Sakura's strengths. For example, her ability to heal is something that is often underestimated. In a team, healing abilities are extremely important, but they may not value it as much. Also, Sakura might not be as vocal about her abilities as the others, so they assume she is weak.
One reason could be that they are comparing her to the more physically powerful members like Naruto and Sasuke. Naruto has the Nine - Tails' power and Sasuke has the Sharingan. In comparison, Sakura's initial abilities seem less flashy. Another reason might be that they haven't seen her full potential yet. Maybe she hasn't had a chance to show off her improved combat skills or her advanced medical ninjutsu in front of them.
Perhaps they are basing their judgment on her past performance. In the early days, Sakura was more of a damsel in distress. She often needed to be rescued. Also, her fighting style may not be as aggressive as Naruto's or Sasuke's. They might misinterpret this as weakness. However, they are not taking into account her determination and the progress she has made in her training.
Maybe he's frustrated with her lack of progress in combat compared to him and Sasuke. He could also be in a bad mood and say it without really meaning it.
One reason could be her importance to the team. Sakura might have some special skills or knowledge that they don't want to lose. So they protect her to ensure the team's overall strength.
Well, it could be that in this fanfiction, Naruto has some sort of hidden agenda. He might call Sakura ugly as a form of reverse psychology. He could be thinking that by doing so, she'll prove him wrong and become more confident. Or perhaps there's some miscommunication or misunderstanding between them. For example, Naruto misheard someone talking about Sakura in a negative way regarding her looks and he wrongly believes it, so he says she's ugly.
One possible reason could be Naruto's out - of - control power. If in the fanfiction Naruto has some new and extremely powerful abilities that he can't fully manage, Sakura might see him as a monster out of fear. For example, if he accidentally causes destruction while trying to save the village.
Perhaps seeing other characters being honest about their own weaknesses inspired her. If she saw that they were able to grow and improve by accepting their flaws, she might have decided to do the same.
One possible reason could be a powerful enemy attack. Maybe she was caught in a fight against a much stronger foe who used a deadly jutsu on her. Another reason might be a self - sacrifice. Sakura could be in a situation where she has to give up her life to save her teammates from Team 7.
One possible reason could be that the fanfiction misrepresents Sakura's character. Maybe it portrays her in a way that she would never act in the original context of the series, which makes her seem out of character and thus causes dislike. Another reason might be that the fanfiction focuses too much on other members of Team 7 and sidelines Sakura, making her seem less important or relevant than she actually is in the story.
It could be because of a dark curse. Suppose Naruto was cursed by an ancient evil. This curse slowly ate away at his sanity and made him view his friends in Team 7 as threats. As the curse took over more and more, he finally reached a point where he killed them. However, this would require a lot of backstory and character development in the fanfiction to make it somewhat believable.
Sakura could prove them wrong by showing her medical skills. In many Naruto scenarios, her ability to heal the injured is crucial. For example, she was able to keep many ninjas alive during battles. Also, she has been training hard in combat. She might challenge one of the members of Team 7 to a spar and show her newfound strength and skills.
One reason could be jealousy. If in the fanfic, Sakura gets closer to someone else Naruto cares about, like Sasuke in a different way than he expected, it might make him hate her. For example, if Sakura teams up with Sasuke against Naruto's wishes in the fan - created plot.