I'm a fan of the High School Musical movies, meaning I love the first one and could go without the second and third on my movie shelf. However, what I loved about Sharpay in the first movie was the scenes with her and her brother, Ryan. They completed each other. For example when it was their turn to go audition for the spring musical, Ryan starts to walk ahead but Sharpay stops him and walks in front of him. He let's her, but rolls his eyes behind her back. Without him, it was just be a self-absorbed Diva being non- humorously pushy with everybody else.
Which is what this movie was pretty much about. They tried to have her co-star be an adorable puppy, but it didn't grab the audience's heart or attention. In fact, I honestly could only watch about thirty minutes of this movie before leaving the room.
Side character spin off movies or television shows rarely work out. They completely flop if you don't include the things that made them awesome to begin with. In this case, her brother.