In my personal headcanon, Derpy is the element of innocence. She represents the childlike purity and wonderment that can be found somewhere inside of us all. As a small filly, she was often heavily bullied, tormented, and ostracized by other ponies because of her clumsy nature and skewed eyes, and ended up spending most of her time, playing with bubbles. They were silent companions that filled her with joy, and never once judged her. Her cutie mark represents her unconditional love for everypony, and her innate desire to help and bring happiness to those who are similarly emotionally wounded as she is.