Mild tends to describe someone's disposition. Depending on that context, the description can be positive or negative. For example, in the 1950s TV series The Adventures of Superman, Superman's alter-ego Clark Kent is described as
Mild-mannered reporter for a great Metropolitan Newspaper (Wikiquote)
Mild in this case is a neutral or positive quality, indicating that he is gentle and kind. In contrast, the quality would be undesirable if someone were being criticized for passivity:
Henry was the oldest parent in the house, fifty-eight, a psychiatrist, a mild man who let life happen to him, let the people he loved talk him into things -- like cell phones or children or trampolines. (The New Yorker)
The issue with using it as a recommendation is that it often seems like a permanent trait rather than a cultivated one. Or, as Bill Louw and Marija Milojkovic put it in their corpus analysis of the word "mild" (in Corpus Stylistics as Contextual Prosodic Theory and Subtext),
Not only is 'mild' not very frequent when it describes people, but it also refers to a permanent personality trait, a habitually shown one. ... One may strive to be modest or resolute, but it is hard to see mildness as a character feature that is cultivated. Also, mildness seems to be a trait that exists in relation to other people, or is shwon in their presence. It is the opposite of violence, whether real or pretended, but it must exist in relation to others.
Whether mild is indeed set in stone or changeable is more a matter of psychology than usage; just know that it tends to be observed rather than instructed.
Even if it were possible to instruct someone to "be mild," but you leave your own recommendation subject to the interpretation of that individual. Can they really make themselves more mild? Some readers would disagree. Is making oneself mild desirable? That depends on whether they see passivity as a positive quality, and whether the situation calls for it. (Clark Kent the reporter should be mild; Superman needs to not be.)