This is a design question that does not have a definitive answer. Some people feel that your bath mat should match your shower curtain in order to create a cohesive look in the bathroom. Others believe that your bath mat can be a different color or pattern than your shower curtain in order to add some visual interest to the space. Ultimately, it is up to you to decide what look you are going for in your bathroom and what colors and patterns you think work well together.