If someone calls you mulish, they don't mean that you look like a mule; they mean that you are as stubborn as a mule. Act more open-minded, and no one will call you mulish.
You might know from the mulish expression on your sister's face that there's no way you can convince her to give you a ride to school, and a politician's mulish refusal to debate her opponent might end up hurting her on voting day. Mulish behavior is inflexible and opinionated. When you're mulish, you're as stubborn as the famously stubborn mule, an animal that's had this reputation since at least the fifteenth century.