1) She doesn't have a real relationship with the guy. It's entirely based on deception, and they barely interact together in person. In Boys, there is the lie of the fake relationship, but they are still actually together, forming a real relationship. In Sierra, they are barely on screen together. Also, Boys has a consensual relationship, while in Sierra, the relationship is not even consensual on one side.

2) They don't have a deep relationship, and there doesn't appear to be real chemistry or even a basis for it. In Boys, the leads bond over actual meaningful stuff, the death of her mother and the abandonment by his father, and they open up to and share with each other. In Sierra, they talk about stuff like favorite things and what animals they would be, and that's supposed to be enough. Sierra and Veronica actually have a deeper, more real relationship than Sierra and Jamey.

3) Sierra does something malicious and cruel towards the end, for very little reason, and she's entirely in the wrong. Then Veronica forgives her, but there's no basis for it. Sierra doesn't even really apologize, she sends her a song about being a sunflower who wants the boy to pick her, not something showing contrition to Veronica.

4) Let's face it, looks matter, in a movie like this. A more serious movie may be just about the storyline, but in a light rom com, it generally helps for the leads to be attractive. Sierra has a somewhat cute face, but she's fat and not particularly attractive. And she's not even attractive on the inside. She's smart, but she never really displays good character or a beautiful inside, in fact she does the opposite.

So yeah, it's not really what I expected, and I normally enjoy these kinds of movies, but didn't really care for this one.