In the book: Elena is very prim, self-righteous, passive aggressive, and judgmental. She especially disapproves of anything that doesn't align with her idea of living the "perfect" life. Her privilege as a rich white woman often seeps through her gestures and is more of an undercurrent than a denotation. Because of this, her actions — helping Linda maintain custody of Bebe's baby, looking into Mia's past, and trying to establish the quintessential family dynamic to the chagrin of Izzy — are rooted in her genuine belief that what she does is right. She doesn't lack conviction; she just lacks awareness of the ignorance embedded in her conviction.
On the show: There's no beating around the bush with this one: Elena is a lot more insufferable in Hulu's take than in the book. It feels as though she has less conviction and grapples more with convincing herself of her firm beliefs and morals. And her privilege is so thickly displayed, it's hard not to grimace any time she speaks.
She's also messier in the series than in Ng's work. The scene in which she tries to bribe Bebe into dropping the case against the McCulloughs doesn't happen in the novel. The book version of Elena is too smart and too calculated for that. She'd know that would throw a huge wrench in the custody battle. She certainly wouldn't have gone out to dinner with her ex, Jamie, either. Nor would she sip through an entire bottle of champagne with him and invite him back to her hotel room, but that happens on the show. Making him a New York Times journalist who reconnects with Elena in the adaptation does spice things up. And watching him retort Elena's attitude is incredibly satisfying ("It's not my job to make your life bearable" is one of the sharpest lines uttered thus far). Granted, in the book, Jamie never reappears. He goes off to fight in the Vietnam War, and Elena never sees him again.
All of these character adjustments are made to up the dramatic ante, which is understandable when you're translating a novel to a script. But, in Elena's case, some of her decisions deviate from the core of her ethos in a way that cheapens her turn of mind.