Remember the scene where Mia is asking her car keys to Sebastian at the end of the party, and there is only Prius keys in the box?
Why are there so many Prius keys in the box? Is it just for fun, or is there a real reason?
I have not yet seen the movie myself, but as far as I understand its nothing more than product placement.
"As well as scenes of the film’s star Mia (Stone) being stuck in LA traffic behind the wheel of her Prius, one of the most memorable and talked about moments in the film is when she & Sebastian are trying to her parked car after a Hollywood party. When asked by the valet what car she drives, Mia’s response of a Prius is met with a wry smile and a panning shot of a line of parked cars that are almost all Priuses, reinforcing the stereotype of the eco conscious Californian." Source
My conclusion is, that this is a common case of product placement where you need a memorable scene, recognisable product (in this case parking lot full of Priuses) and a dialogue-environment to show it as natural part of film. I did not find online scene for this, so I don´t know if the scene was actually memorable or fitted in the movie, but according to your statement and source above the product was clearly recognisable and the environment was supported by the claim of eco-conscious California.