Deliberately writing a lifeless character in the hopes that all teen girls everywhere would project into it seems like a bad plan. A lot of book series that do well allow you to empathize and somewhat project yourself on to the main character while still featuring interesting characters. You can have first person narrative characters who are interactive, witty, entertaining, and emotional.
The reason Bella appears to be a flat character is because Twilight is wish fulfillment pablum about girl meeting absurdly handsome and rich magical boy who will then make all her decisions for her and she will never have to do anything ever again. And because of this idea, Bella is entirely dependent on other people for everything. She can't be happy unless she's with sparkle-ass the vampire. She apparently does risky motorcycle racing not because she likes to, but because it reminds her of how fast sparkle-ass can magically run. She treats everyone around her poorly not because of anything they've done or not done to her, but because they aren't sparkle-ass. She has no will, no agency, in the story whatsoever, but everything mysteriously revolves around her for no reason.
And that's why we get the real problem with the writing. It's not that you can't ever have flat characters in a novel, even in major roles. Asimov pulled it off multiple times in Foundation, but you didn't notice the characters were flat unless you took a hard look at the book. But in Twilight, there's nothing to hide that Bella is a flat character. There's no shading, no effort to make her even a tiny bit of a person. Everything she likes is either obviously supposed to parallel her "great love story" or is incredibly cliche stuff that everyone likes, or is never given an explanation of what about a thing she likes or dislikes. She doesn't get along with her parents, why? She didn't like arizona, why? She refuses to even consider going out with the ab-tacular werewolf, why? She likes sparkle-ass, for the love of god, WHY?! Oh right, because he's pretty. Ugh...
TL;DR: The problem with Bella isn't that she's a flat character, but that she's a very badly written one that is devoid of ANYTHING to even qualify her as a character.