I don't know that I'd consider Bella wimpy. In Twilight, when she was being whisked away after the baseball game by the Cullen's because of James, she insisted on going home first to ensure Charlie's safety. Then when James called her on her cell telling her he had her mother as hostage, she immediately went to try and rescue her mom at the ballet studio.
In New Moon, she raced with Alice to Italy to save Edward even though doing so would put her life at risk because of the Volturi.
In Eclipse, when they had the standoff with Victoria, Riley, Jacob and Seth, Bella cut herself to cause a distraction because she couldn't stand being useless. Her aim was to be of help and hopefully keep Victoria from killing any of them.
Breaking Dawn is ALL about Bella's selflessness. She carried a baby that was nearly killing her. The birth actually did nearly kill her. Then she fought to learn how to defend all the people she loved with her special shield power in a short amount of time. She made plans for Jacob to escape with Reneesme during the battle so that she would survive.
Hmm, she sounds rather strong to me. Do I think these books are the best literature I've ever read? No. But I have to say they have been highly enjoyable and entertaining.