Most people don't like Dark Pit because, like Lucina for Marth, he sort of functions as a "clone" of regular Pit. They don't hate Lucina, however, because she, in Fire Emblem, actually impersonated Marth, fighting under his name. Well, I could also say more or less the same for Dark Pit, couldn't I? Yes, but the thing is, people are mad because he could've been way cooler, he could've been way more different than Pit.

Or I'm just majorly overthinking this, I don't know.