I don't think she was well written. There's a difference between a divisive character and a character you are supposed to hate. King Joffrey in ASoIaF/GoT was intended to be utterly repulsive and a horrid person and in the show adaptation Jack Gleeson knocked it out of the park.
Sera doesn't cone across to me as being written to be intentionally unlikable.
See but Joffrey doesn't really register for me either, I know what he is for the most part, so I don't really experience anything. I don't think you could ever find me going "Oh Joffrey is such an X" because I feel confident enough everyone else will have an exactly similar opinion and there is nothing worth discussing.
But again, "hate," if you just "hated" Joffrey or Sera you wouldn't bring them up at all. I don't think most people realize it but when they discuss something in any context, whether negatively or positively, it indicates a degree of interest. I don't actively think or contemplate "hatred" of a serial killer, it's just such an obvious association of dislike or un-interest that again there is simply nothing to add to the conversation.
Obviously this kind of thinking leads to considerably more complicated moral and ethical implications but frankly I associate commentary, positive or negative, with being challenged in some way. A truly hateful or hated character wouldn't produce the same response.
Basically, if she was really, genuinely, unlikable, by everyone, this thread probably wouldn't exist. If she's just such an obviously horrible, ego-tistical, ew type of character, why are you even here? Wouldn't you want to get away from Sera and as much Sera material as quickly as possible? Why ruminate on hatred of all things?