Some self-proclaimed Ran-haters don't like her for her personality traits/Gosho's use of her as a character. My experience is that most vocal Ran-haters fundamentally hate her because she is blocking ShihoxShinichi and thus come up with proxy reasons to justify their Ran hate. Usually the first sort of Ran-hater also finds Kazuha sort-of annoying as well because she is effectively Osakan Ran, just enhanced. The second type typically ignore or like Kazuha, and support KazuhaxHattori because that pairing isn't "in the way" of another that they like. Of course, this is just generalizations from experience here.
Saying she is unrelated is ignorant of the plot. Ran is primary motivator for Conan wanting to return to Shinichi. Also recall Ran is the reason that Vermouth has decided to sort-of-help Shinichi in the first place. Not to mention Ran's testimony was vital in helping Conan figure out Akai was FBI. Ran also saved Haibara more than once, including the beach where Haibara collapsed, and of course Vermouth's showdown. She also helped Haibara sort out her feelings and begin to get over the death of her sister. Ran does things for the plot. She isn't Shiho Miyano, creator of APTX, full of secrets, hunted by the Org, but Ran has her place in the character lineup.
Everyone has their character likes and dislikes. I don't care for Kaitou Kid, but I don't go off on him in every single thread (this thread is built for the purpose of expressing dislike but "Your favourite scenes in DC" not so much). I would like to avoid a flame war, which means we need to keep the discussion intelligent, civil, and confined to the proper venues.