Parking in Hermosa without a permit can be difficult in the summer, especially as you get closer to the beach. If you don't mind walking a bit, you can frequently find free spots in residential areas a few blocks east of the beach, off of Valley/Ardmore Streets. The city itself is very safe and as long as you don't leave any valuables in sight, you don't really need to worry about someone breaking into your car.
You do need to pay attention to the street sweeping signs in the residential areas. Typically they will post no parking for one day of the week, for either 2 or 3 hours total.