The antonyms for the term "more nonserious" can be "more serious," "more grave," "more significant." These antonyms are used to describe situations or people that require a serious approach. Seriousness is the quality of being important, significant, or critical, whereas nonseriousness is the opposite.
For instance, if we take an example of a conversation, if one person is being nonserious and joking around, the other person may respond by being more serious and discussing the topic more gravely. Likewise, if a situation is nonserious, people would take it lightly, whereas if it's of significant importance, they would take a more serious approach.
Therefore, the choice of words depends on the nature of the situation, and we must use them accordingly to communicate effectively.