One of the primary themes of the novel is carelessness of people in general, but especially the upper classes. There are numerous examples of shallow, careless behavior throughout the novel. There is the description of guests behaving with a "simplicity of heart" as if they were in a an amusement park. There is the car accident scene at the end of Chapter Three. There is the behavior of party guests at Tom and Myrtle's flat in Chapter Two, and of course, there is the ultimate condemnation of behavior in the novel when Nick decries Tom and Daisy in Chapter 9, "They were careless people, Tom and Daisy—they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made. . . ."
Jordan's behavior is just one more example of careless people who don't consider the consequences of their behavior. Remember, Nick is in search of a world at some sort of "moral attention." The people of the East certainly are not a part of that world.
To me, Jordan Baker has always embodied the indifferent or apathetic character who was so indicative of the twenties. There are many qualities of Jordan that are just typically "flapper". The flappers rebelled against their parents in a way that had traditionally never been done before... especially by females.
Jordan also uses people to get what she wants. Obviously, in cheating in the tournament she was seeking a higher score and then pay-out. She uses the Buchanans' home as a hotel, and she uses Nick as a temporary boyfriend to suit her needs when she has needs. She cares nothing for his feelings or for the Buchanans finances (not as if they need her concern anyhow). She is rude. The car's top is just a simple example of a greater lifestyle habit within Jordan Baker.
Some of the most important character traits of Jordan's, in terms of the story, are her dishonesty and her willingness to use people and forget the consequences. Both of these episodes reveal the fact that she does not have a really strong moral character, particularly in the case of the golf tournament.
Her leaving the car out with the top down suggests, as do many of her other behaviors, her absolute disdain for anyone and anything. She feels that people have been placed in her life for her to use them to get whatever she'd like, and so why would she feel any remorse about possibly ruining someone else's car?