Instead of "dumb," consider:
* "Silly" - "That was a silly mistake to make."
* "Foolish" - "It was foolish of him to go out in that storm."
* "Naive" - "She was naive to believe his promises."
* "Unwise" - "It was unwise to invest all your money in one company."
* "Unintelligent" - (Use with caution, as it can still be perceived as offensive.)
Examples of using "dumb" in a non-offensive way:
* "The TV show had a dumb plot." (Referring to the show itself, not a person)
* "That was a dumb idea!" (Expressing frustration about a plan)
* "The old phone had a dumb design." (Referring to an object's functionality)
Remember, it's always best to choose words that are respectful and avoid using language that could be hurtful.