Why don't I dislike Oota's behaviour? Isn't that bullying? I have thought of some reasons as to why people tend to look over:
1) Oota's objective is to see a different expression on her face, not to destroy her life or something.
2) His pranks are... kiddy stuff. Whenever he plans to do something more serious he pulls back just in time.
3) On top of that, he never makes Kashiwada an object of ridicule. If anything, people make fun of him for his obvious crush and the very weird way he shows it.
4) There's also the fact that the supporting cast, especially Oota's friends, tend to be supportive of the two, often acting as mediators.
So, while petty, targeted pranks are a sign of bullying, the intentions, gravity, consequences and context of the pranks really don't align with bullying.