Mañanero literally means 'morning person' or something related to the morning. However, in many Latin American contexts, especially in urban slang, it's commonly used as a euphemism for 'morning sex'.
In this song, DEKKO asks, "Dime si quieres que yo sea tu mañanero" (Tell me if you want me to be your 'mañanero'). Given the romantic and intimate context of the lyrics, it strongly implies the latter meaning, making it a playful and enticing word choice that adds a layer of modern, colloquial Spanish to the song.
Have you ever just wanted to ask someone to be yours, no matter what? That's exactly what DEKKO is doing in "Mañanero"! He's laying it all out, asking his love interest if she wants him to be her "mañanero" [morning person/lover] and travel the "mundo entero" [entire world] with him. It's a sweet, direct plea for a deep connection.
He's not just talking about travel, though. DEKKO wants a lasting bond, saying he'll "nunca te voy a querer soltar" [never want to let you go] and that he's the one who "jamás te va a olvidar" [will never forget you]. He even admits he "era un fuckboy pero tú me has vuelto un caballero" [was a fuckboy but you've turned me into a gentleman], which is a pretty big declaration! It's a song about finding that one person who changes everything and wanting to commit completely, even if it means facing his "karma contigo" [karma with you].