Calladito is the diminutive form of callado (quiet or silent). The diminutive suffix "-ito" or "-ita" in Spanish often adds a sense of endearment, smallness, or in this case, a slightly playful or ironic tone.
In the song, the lyric "calladito te ves más bonito" translates to "you look prettier/better when you're quiet." It's a common, somewhat sassy phrase in Spanish, implying that someone's words often ruin their appeal. The use of calladito makes the phrase more informal and emphasizes the desire for the person to remain silent.
Have you ever broken up with someone, but then you miss them like crazy after a few drinks? That's exactly what Kenia Os is singing about in "Mi X"! She admits that "Cuando me doy un trago, dos o tres" [When I have one drink, two or three], all those wild nights with her ex come rushing back, making her want to get back together.
But here's the kicker: as soon as he opens his mouth, everything goes wrong! She hilariously says, "Es que la verdad, calladito te ves más bonito" [The truth is, you look prettier when you're quiet]. Even though they're amazing in bed, like "rock-star[s]", they're really "como agua y aceite" [like oil and water] and "nieve y verano" [snow and summer]. So, even though she sometimes wishes he wasn't her ex, she knows deep down that they just don't work out.