@Kayriel People are more pissed about the combination of her being ultra-possessive while not taking the relationship anywhere. Ippo is also a dumbass because of it, but at least he doesn't have a yandere act to go with his neutron-star density.
Her not wanting him to box anymore would be fine and dandy on it's own, but in combination with the above attitude it just comes off as manipulative to the point that she has his trainees report to her. Her feelings may come from a place of love, but it doesn't make them good per sè.
Nobody thinks she's a bad person because she feels some way or another. Ippo's mom never got flak for feeling things about Ippo boxing or never seeing his fights. Kumi actively interferes with something everyone in the audience wants to see, and everyone knows she will fail, so it's just a constant annoyance that occupies pages that could be used to move the actual plot without her making threatening faces.