The phrase "stubborn as a mule" refers to someone who is unyielding, rigid, or inflexible. If you want to describe the opposite of such a person, you may use the antonyms like compromising, yielding, docile, flexible, obedient, and cooperative. A compromising person can accept alternative suggestions, whereas a stubborn one may refuse to give in. Someone who is docile is teachable, which contrasts with the stubbornness of a mule-like personality. Flexibility refers to the trait of being adaptable and open to new ideas, whereas the rigid mindset of a stubborn person may reject fresh perspectives. Lastly, obedient and cooperative persons are likely to work harmoniously with others instead of being uncooperative and rigid.