Ingenua translates to "naive" or "gullible." It's a key word that captures the singer's initial state of mind in the song.
Ha*Ash uses this word to describe how she was initially fooled by her ex-lover's charming facade, believing he was "the lottery." The word highlights her regret and the realization that she was too trusting, making it a central theme of the song's narrative about deception and heartbreak.
Ha*Ash open with a guy who seemed perfect: he opened doors, sent flowers for no reason, dressed sharp, and was "más encantador que Romeo en el balcón" (more charming than Romeo on the balcony). Then the details start falling apart. The flowers he sent "me espinaron nada más" (just left her with thorns, nothing else), and the car he used to impress her wasn't his: "No era tuyo era de tu papá" (it wasn't yours, it was your dad's). By the chorus she's landed on the line that gives the song its title, "Eras demasiado bueno para ser verdad" (you were too good to be true), and she means it as an insult.
The best twist comes at the very end. For most of the song "demasiado" (too much) describes him: his fake charm, his borrowed car. Then the last lines flip the word onto her, "Que yo soy demasiado, demasiado para ti" (that I am too much, too much for you), turning her list of his lies into her own verdict that she was too much for someone that fake. She even lands a jab that only works in the original language: "Tú apenas hablas español" (you barely speak Spanish), calling him out as fake down to the language he claimed to share with her.