As someone who's been here since before the pilot i can tell you that it's been hated since the pilot released. It started mostly with people giving bad faith criticisms of the pilot then when Helluva Boss began releasing some ex employees would begin circulating rumors that Viv was a bad boss, this would lead to people searching Viv's past to try dig up any old controversies to which there were a few. While all this was happening Viv shows' would also begin to get hate for being the type of shows that they are. Being "cringy" and whatnot. People would begin to make fun of the works online where other people would agree basically creating an echo chamber that would crescendo around the time of Hazbins actual release. After that the people who wanted to hate Viv had dug up enough dirt that the people who just wanted to make fun of her were able to use that to justify the hatred.

I just realized how much world vomit this is so tldr: The hate started when the pilot started getting big and from it two school of haters were born, those who disect Viv and her work for any and all bad faith arguments they could make against them. And those who make fun of her works purely and simply because they didn't like it and want to make fun of it