Guapetón is an augmentative form of guapo (handsome). It implies someone who is very handsome, often in a charming or even slightly boastful way.
In the song, the lyrics state, "El más risueño y no el más guapetón" (The most cheerful and not the most handsome). This suggests that being cheerful is more attractive than just being extremely good-looking, adding a playful and relatable twist to the idea of attraction.
Have you ever been completely smitten with someone, even a little obsessed, but also super nervous to approach them? That's exactly where our singer is in Los Amigos Invisibles' "La Que Me Gusta" [The One I Like]! He sees "la más linda del lugar" [the prettiest one around] and knows he has to try, even if he's scared of just being "uno más" [one more] admirer.
He's trying to convince himself (and maybe her!) that he's a great catch, promising to be "mejor que la anterior, no rico pero encantador" [better than the last one, not rich but charming]. He's a "mango" [a hottie, literally 'mango'] with good humor, no crazy habits, and no weird issues with his mom! He's so excited when she finally smiles at him, leading to that fantastic moment where he realizes, "¡Ayyayay! le gusté a la que me gusta" [Ayyayay! The one I like, liked me back]! It's a fun, upbeat story about taking a chance on love and getting that happy ending.