We don't know or care why or how it happens because that's not important. I think it's simply to demonstrate that she's not well-to-do. There are lots of movies where people are seen diving "beater" vehicles. This is no different. They probably thought that kind of damage was, visually, more humorous.
The damage doesn't prevent the car from working, so while others might find driving around in a dinged-up vehicle to be embarrassing, you see a lot more of that in lower income areas because economic reality will trump social and cosmetic concerns that might win out with people with greater means and insurance coverage.