Mundão is an augmentative form of the word mundo (world). The suffix -ão is used to indicate something large, grand, or sometimes even overwhelming.
In this song, Mc Brinquedo uses mundão to emphasize the vastness and difficulty of the world he lives in, especially when he sings, "Tá difÃcil viver nesse mundão" (It's hard to live in this big world). It's a common informal term in Brazilian Portuguese that adds a touch of colloquialism and emotional weight to the lyrics, making it more impactful than just saying mundo.
In "Mundo Melhor," which means "Better World," Mc Brinquedo and Mc Kevin reflect on the challenges of modern life. They describe the world as a difficult place to live in, a constant "batalha" (battle) and "correria" (rush). The lyrics express a deep desire for a better world, one where people have more "disposição" (willingness or motivation).
Despite the struggles, the artists also find a sense of personal peace, repeating the line, "minha vida tá de boa é isso que importa" (my life is good, that's what matters). This creates a powerful contrast between being content with your own life while still wishing for improvement for everyone else. It's a song about finding your way through the hustle, keeping faith that "Deus vigia" (God watches), and dreaming of a better future.