Blunderheaded, meaning foolish or stupid, can be substituted with various synonyms to convey the same message. One alternative is "clumsy", which suggests a lack of coordination or gracefulness. "Inept" is another synonym, indicating a lack of skill or competence. "Foolish" can also be used to describe a person who makes blunders due to a lack of good judgement. "Unthinking" implies that a mistake was made impulsively, without proper consideration. "Careless" suggests a lack of attention to details and a disregard for consequences. Regardless of the synonym used, the underlying meaning remains the same - a person who makes foolish mistakes or shows poor judgement.