@Drake98
Well, not exactly. I've read quite some stories where I hated or disliked the main lead. (I like flawed characters, but toxic and super selfish/hateful characters are super hard to connect to.) Especially if there's no character growth. Trashta would be such a case, because no matter how she glorifies her POV, as a reader you can read between the lines and see how fucked up her actions are, how shallow her thoughts. If the author writes well - and the author of Remarried Empress certainly does - you can really pull off that disparancy. That's however based on the assumption that everything would be told and trashta doesn't suddenly turn into a completely unreliable narrator.
Just because a character is the main lead doesn't mean the reader automatically has to sympathise with them. It makes it easier, yes, because more people allow themselves to believe trashta blindly without seeing her actions, not reading between the lines, because as you said, we like to relate. And we like to relate to the good people.
But at the end of the day, trashta is the villain of this story because she behaves like one. If she behaved differently, she wouldn't be the same person who could have character growth and learn to have and appreciate her new life.
And it's not like there's a villain in her story causing her inevitable ruin, either. The empress is cold, sure, but mostly she ignores trashta and let's her be. That money hungry grandfather of the first child, who would make a great opportunity for personal growth and such, can't actually ruin her either without ruining himself and his family. Any ruin she'll encounter is of her own making.
So yeah. If it were trashtas story, it would most likely be a tragedy. Or tracomedy or satire, how you spin it. More people would sympathise with her, but not everybody. Especially not after the part of the duchess, where she destroyed that family for fickle reasons.
You may sympathise with them more as the main lead, but you certainly don't have to like or even love them. But I agree, the POV really does make a difference for sympathy points, it just doesn't automatically make a bastard character into a beloved one and it never will.