The phrase "more light-minded" refers to someone who is cheerful, carefree or relaxed. The antonyms for this phrase could include phrases like "less light-minded", "heavy-minded", "serious-minded" or "grave". A person who is less light-minded may be more reserved, introspective or contemplative. They may be more focused on serious matters and less interested in frivolous activities. A heavy-minded individual may be weighed down by responsibility or burdened by worry. Those who are serious-minded tend to be more analytical, logical or thoughtful. Lastly, someone who is grave may be rather solemn, dignified or formal, showing little inclination towards fun or levity.