Is "mild person" positive or negative characteristic?

Solution 1:

I think you might be thinking of meek, meaning quiet and gentle but may also connotate someone as also being submissive, which can allude to being negative; however, a mild disposition can be negative or positive, depending on how it is perceived. A dog with a mild disposition is typically thought of as being a great attribute but a "mild" president with views on terrorism policy may be viewed as negative. You may want to think of other words that might be a better fit for your slogan - e.g., Other words to consider might include "humble," "kind," "gentle," etc. Also, think about combining stronger words to your slogan (e.g., "Be Strong, Be Gentle".. or "Strength is Humility, Peace and Serenity"

Solution 2:

"Mild" isn't used to describe people enough for it to have any positive or negative connotation as the adjective "mild" itself is generally neutral, only having connotation when paired with a handful of nouns, like "weather."

As regards "people," some might infer that "mild" means calm or absent any outlandish qualities, but others might infer that "mild" means boring or bland. It would depend on context.

In your context, saying "Be mild" as a slogan on a poster, it's clear that you mean it positively. That doesn't mean that people, understanding what you mean, won't nevertheless scoff at it, especially people who aren't "mild," meaning with such a poster, you'll likely only be preaching to the choir.

Maybe you'd be better off using the adjective "mild-mannered," which does have a positive connotation as it approvingly describes one as being gentle, kind, and polite.