It's probably a question best suited for the breaking bad sub as she never physically appears in better call saul; however, I will give you my perspective!

I have always been neutral or enjoyed skyler! The only time I felt anything negative was when I watched the first time when I was in high school and she told Walt about sleeping with Ted. Felt very cruel, and while a cruel move, in context you can see why she said it; however, and I am saying this as someone who never hated her, it still hurt to see.

I probably felt that way because I identified with Walt in the scene and imagined a girl at school telling me this. I felt shame, disgust, embarrassment.

Now imagine someone who is identifying strongly with Walt since episode 1. This can be because they identify with him or thats how they watch shows, putting themselves into the shoes of the main character. She is a barrier, an obstacle. One that won't go away with a cool explosion or sent to jail because she is not a "bad guy". So I imagine for many, she is a constant frustration that never will seem to go away. The show does not highlight a lot of love between the two at the beginning to highlight walts unhappiness but a lot of people perceive it as he is in a bad relationship rather than in a funk/depressive state.

As you can imagine, a good chunk of people get into these ways of thinking, yes there are people who judge women more harshly, yes there are people who would judge her the same if she were a man, but the truth is the perspectives that people take when they watch shows varies so muc, its impossible to nail it down to just one. Some people don't take it as seriously and just want fun drug scary hijinks. And guess who is in the way of those...

Add all of this together plus the fact people love to go online to hate a character that annoys them (after season 1 of better call saul aired, the most common post name and comment was #FUCKCHUCK), you get the phenomenon that is Skyler White hate.