Yeah I get that a little bit, but try thinking about it in a different way. Put yourself in her shoes she was a girl that was being picked on just like Shauna. She finds a guy (who her brothers introduced her too) that she learns was protection a girl just like her and wishes she had someone like that in her life too. He becomes her hero and wants to learn more about him, and then she finally gets a chance to meet said hero and sounds like a silly fangirl (don't think of her as a stalker think of her as a fangirl). Once you get to know her a little more she explains why she is so interested in the MC and Shauna and sounds less like a creepy stalker and more like a fan who wanted to learn more about her hero. Who would have liked to have someone like the MC in her life to make her feel safe too.
While Penny is not as sexy as Shauna they are very similar, you can also make a point that Shauna was like a stalker too. She forced her way into the MC life because he protected her when she needed it most. She followed him around, inserted herself into his life and even spied on him in the shower. Both girls are insecure with their bodies, their personalities, nether felt they would be loved by anyone else or that anyone else would find them attractive. Penny is basically Shauna just a skinnier version with less curves, and more blah hair. I love both Penny and Shauna since they both remind me of my self at a younger again. Insecure, hated my body, and thought I would never find anyone that would love me for me.
As always I can't change your minds about a character but I like to try to get you to look at them from a different perspective.