Maybe Umbridge just strikes too close to home for most people, which is why people hate her so much more than others. Everyone's had an Umbridge in their life. However, J.K. Rowling certainly primed the series for Umbridge long before her appearance. Rita Skeeter is 110% evil, and Fudge certainly starts to move towards there by the end of the fourth book. Snape is actually ridiculously unfair in the first few books, disproportionately so, and especially in the Prisoner of Azkaban, where he's actually the main villain of the story. (Of course it's chalked up to him being blinded by his hatred for Sirius/Remus due to their treatment back in school, but he is still entirely unreasonable.)