I could sum up the reason for my rating pretty succinctly with the statement, "If you wanted me to root for the fake dating to become real dating, you shouldn't have gone so hard on the love interest being such a fuckboy," but I honestly think it's more complicated than that.
Every time Mona finds fault with Lucas' actions, it's because she's taking it as a personal insult—that he makes her look stupid, that he's hurting her brand, that he's careless of her as an individual. I, as the reader, on the other hand, saw it as a pattern of behavior. This dude says over and over again that Mona is his dream girl, that he admires her spunk and wants to be with her. And yet, he repeatedly does whatever the fuck he wants without regard for her feelings and well-being. That's not someone who's careless of Mona; that's someone who's careless of everyone except himself.
The fact that I am told that he's hot and he and Mona have great chemistry—even the fact that he and Mona have things in common, like their dead dads and their controlling and famous moms—is not enough to convince me that Lucas is a worthy romantic prospect. Nor is his protectiveness of her against outside threats...because, hate to break it to you, but that protectiveness only counts if it also protects her from being hurt by Lucas' bad behavior, too.
Kai may not be the right love interest either—famous people can't date non-famous people, yadda, yadda, whatever—but I like him as a person. I find his family and his history and his perspectives interesting. When he's on the page, I'm engaged. Anytime Lucas is on the page, though, I'm bracing for yet more gaslighting and yet more manipulation, not just from him toward Mona, but from the author toward her readers.
I do not buy that Lucas is a good guy. I do not buy that he'll change his behavior because she's the one. Mostly, I just think Mona got stuck with some Max Davenport readalike and doesn't know it yet, and all the reality TV-like hijinks and fun moments with Kai and Mona's family don't make up for the fact that I think she deserves better.