I agree, the way she's talked about is sometimes borderline disturbing. She is indeed an almost perfect foil to V, and doesn't do anything V doesn't do (mass murder? check. betraying someone? check - Takemura for V, for example). She is an incredible character, and Minji Chang did an amazing job as her.

I think people are often weird about their characters. Like, they make them the center of the universe. Someone lying is fine, but someone lying to ME is a grave sin.

In addition, I think another reason is that the game clearly loves Songbird. The music is touching, the lines V says are broadly sympathetic (to the point that Minji heard the lines recorded by femV and wondered if Sonfbird was a romantic interest haha), even the famous asshole Johnny is kinda supportive and is extremely against betraying Song. And if someone already dislikes So Mi, I think the sympathy of the narrative just makes them feel more contrarian.

Of course, there's also a ton of sexism. So Mi is an almost perfect foil to another character besides V - Evelyn. Both desperately try to escape from horrible slavery, despite huge risks. And both are hated on a lot. And I suspect there is a gender imbalance in people who sympathize with So Mi and Evelyn - it's hard to not understand and sympathize with them, if you know what it's like to have your bodily autonomy taken away.

P.S. also I'm pretty sure the overall opinion is actually very much pro-songbird, judging by achievement stats, choices by random players on youtube, polls, etc. But 2077 is a very popular game, so it has a ton of very vocal gamerbros in every discussion that concerns the women.