Anyone who likes Sera enough to spend time with her shouldn't be baffled by this. It's incredibly hard to like her. She's pretty stupid, emotionally unstable, indecent, cosmopolitically inexperienced, and throws around the unfiltered, misinterpreted predjudice a five year old picks up from eavesdropping on their parents.
My take is, that many subconsicously expect all characters to make sense. They expect them to have a thought-through agenda to their possible "unpleasantries" & nasty habbits and whatnot, or otherwise they will not accept them as "proper characters you are not supposed to love" on the spot, but rather just badly written ones. For the same reason IMO many people were repulsed by Morrigan and confused by all the mystique to her being an evil witch, instead of just accepting that, underneath all this, she was simply intended to be emotionally immature.
Personally, I love Sera. My male Inquisitor has a very unique relationship to her, which is like tutoring an adolescent teenager. Except unlike Tali'Zorah, not everything she says & does is necessarily cute. But that makes the moments where she actual proves to be "better" after all, ever so valuable.
P.S.: Also she randomly makes and laughs about her own fart jokes whilst wandering through the desert. In a game where every character insists on being mysterious and dramatic, that also helped.