The reason why she's in a wheelchair hasn't been revealed. She appears to be a good friend of the 'Shortest Girl', as they are almost always seen together. The two later got into a fight over the pages of Burn Book, but soon-after they are seen together once again, so they presumably recovered their friendship.
Laura doesn't play a major part in the movie's plot. She makes her first appearance She hates a fat Laura, which is seen in a scene where Jessica publicly said that she 'Doesn't hate her cause she's fat, she's fat because I hate her'.