Not all, but most of the people I see who dislike her never actually got to know her because they were put off from the start. Almost everyone I've encountered who has actually played through her relationship comes around to appreciating her.
That's funny, because what I've seen is that most people who like Sera just assume that people who don't like her just didn't bother to get to know her (like they think she's so inherently likable that the other person must have just not gotten that cookie scene or the romance scenes or else they'd adore her), but from what I've seen a good many people who dislike Sera did try to get to know her, did try to raise her approval, did romance her, did do whatever, and still didn't like what they saw.
I personally spent the entire first playthrough trying to understand her point of view and be supportive and diplomatic even when she constantly badmouthed my people, and in the end she still tried to force my Dalish Inquisitor to renounce her religion/culture, then acted like I was the intolerant bigot for refusing to blindly agree with her, even though she was trying to force me to see things her way and never the other way around.
I also saw her ultimatum to a Dalish Inquisitor girlfriend, and saw red. A Dalish Inquisitor can be the perfect girlfriend who always agrees with her, goes with her whims, gives her her way, and endures her badmouthing the Dalish, and in the end Sera will still try to force her to renounce her religion/culture. And do so in a very emotionally manipulative way. Like, "If you loved me, you'd say the elven religion/culture is all false. If you don't, then you're just an intolerant bigot like the rest; you just don't accept me or love me like the rest." But again, Sera is the one who is being bigoted and intolerant by trying to force you to see things her way; Sera is the one who is not accepting you for who you are since she's trying to force you to change then dumps you for being "too elfy" when you won't. Even if you never tried to push the Elvhen religion or culture on her or tried to agree to disagree, it's not good enough for Sera; you have to change into something she finds acceptable, or she'll dump you, all while acting like you're the one not accepting her.
And that's why I don't like Sera. Plenty of people did get to know her and still didn't like her, but ironically getting to know Sera more is what made me hate her more. I just can't stand what a selfish, immature, hypocritical, intolerant little bigot she is.
(And don't give me that, "Well, other Dalish/city elves mocked her for not being 'elfy' enough..." BOO HOO. A) This is Thedas. Everyone has a tragic backstory of being rejected or not fitting in, and plenty of them find the means to understand or accept others. B ) THE DALISH INQUISITOR (POTENTIALLY) DIDN'T DO IT! Sera is punishing you for other elves' treatment of her, and I frankly don't appreciate it. Even if you're the most supportive, open-minded, encouraging friend or girlfriend, Sera still shows that she won't accept you or respect your differences, but still tries to emotionally manipulate and twist it around so you're the bad guy for not changing for her, and I don't ****** have time for it.)
I don't think Sera DOESN'T care about the little people, I think she cares about them in a very immature and selfish way. She reminds me of the stereotype of a young political activist who is more interested in the attention and outrage they can generate than actually following through on their cause.
Yeah, I think it helps that Sera's in-file character description describes her as a "selfish criminal" who "robs the rich, keeps it." In-game Sera makes it clear that she enjoys being with the Red Jennies for self-gain as much, if not more than, social gain. As she tells the Inquisitor at different points, "Fixing the world ought to earn more sovereigns than this. We need things back to normal so I can go play." And "[Stopping Corypheus] Helps me, helps people, helps you. In that order."