Mona is…um…Mona is Mona. That's perhaps the best way to put it really. She's a knockout, bombshell of a woman, and John falls for her when he sees her picture in the newspaper. (For her intellect, we're sure.) When John gets the chance to be President of San Lorenzo, he agrees because it means Mona will be his wife.
Mona's character is an odd combination of sublime sex symbol and frigid monkish sexlessness. She performs boko-maru with John—and it may be sexy, but it's definitely not sex.
And then, even when she's married, she won't be faithful to John be performing boko-maru with him and him alone. As she claims, "I make people happy. Love is good, not bad" (93.26). After ice-nine hits the ocean, John and Mona hide in Monzano's oubliette where the two have actual sex. But Mona is "not interested in reproduction—hated the idea" (118.18).
Her character shows that sex and love are separate. An act of love is anything that makes another person happy even if it turns out to be a lie—as with John and Mona's relationship. Meanwhile, sex is sex and not necessarily an act of love.
That's a kind of depressing outlook, if you ask us. And it's not too surprising that Mona kills herself after they emerge from the oubliette.