Well there were plenty who thought she might be trans when the game came out (she's not, she's just swoll). Bigots hated her initially because of that.

Then there were people that hated the character viscerally because of what she does in the game

SPOILER

Specifically, she kills Joel, the main character from the first game and father figure to Ellie, the main character in this game.

That's fine, actually. You are meant to hate her and want revenge on her. That is part of the narrative and the meta of the game. As often happens when you hate someone, a lot of people then justified hating everything else that's different about them. In this case, her predominant feature that differentiates her is that she is a beefcake. So then people were ripping into her for looking masculine or ugly.

Then, a large number of people were forced to look inward at themselves and face their own reaction and hatred to Abby's actions halfway through the game.

SPOILER

This is because the game switches things up in a major way nearly halfway through the game. You spend the first 40% of the game as Ellie, consumed by grief, rage and hatred, on the warpath to get revenge. You are killing Abby's allies that helped her kill Joel, all while trying to find and kill Abby herself. You put revenge even above the wellbeing of your love interest and the father of her child. But then at the height of your revenge quest, just as you get to the apparent final showdown with Abby.... the story switches to Abby's perspective a few days prior, about the time that Ellie arrived in the city to kill you, Abby. And now you get to meet all of Abby's friends, your friends, the people you had been hunting down as Ellie up to this point. You find out that Abby had her own justification for killing Joel, her own vendetta over the death of a father, just like Ellie. Turns out Joel killed Abby's father when he rescued Ellie at the end of the first game, and so she hunted him down and avenged her father. Now you play as Abby as she deals with mixed reactions to her successful revenge quest, as she befriends a young trans boy and his sister who were fleeing from the religious cult they were born into, as she defects from her own people rather than participate in the mass genocide of that cult, and as she saves that boy and takes him under her care as family, not unlike Joel did for Ellie.

The game makes you feel the rage and intense desire for revenge and then smacks you across the face for it. The message is clear. Hate begets hate, revenge propogates revenge. The only way to find peace, both within and without, is to break the cycle. And delivers this message in a very visceral way. The game forces empathy and compassion on you whether you like it or not. And many people did not like that. Many people were absolutely furious that they were expected to step into the shoes of someone that they hate, to examine and question their own hatred, and to sort out the complicated feelings they have over someone that they condemned to death for their crimes also being a friend, a lover and a savior to other people. They could not or would not process those feelings as the game designers intended, and so they doubled down on their hatred of Abby AND of Naughty Dog/Neil Druckmann for expecting them to.

Then there were plenty that were just happy to hate a game featuring predominantly female characters, a trans character, and with a pro-compassion message.

TLOU2 was a peace of art, as was TLOU1. And the message, while not subtle, is masterfully executed on and makes you truly examine your own impulses and hatred. I love these games.

The character had the audacity to be a strong young woman with a nontraditional build, and not conventionally attractive.

I mean, I hate the character for her actions (early in the game at least).

There aren't "good" reasons to transfer anything from a character to an actor, so it's pretty moot.

Before the game came out we had two major leaks. The first one was that Joel gets killed, and that Abby is the one who does it. The second one was that it features a trans character. Abby being a muscle mommy made those people assume that Abby must be the trans character. So from their perspective, a trans character murders their beloved (and definitely no closeted gay feelings attached) daddy Joel. This fabricated & deluded image became their perfect scapegoat for their fake outrage.