Desmadre is a very popular and expressive Mexican slang word for "a mess," "chaos," or "a disaster." It's the kind of authentic, colorful vocabulary you won't typically find in a textbook!
Literally, it's a combination of the prefix des- (undoing) and madre (mother), suggesting a situation that has gotten out of a mother's orderly control. In the song, the singer says his new girlfriend doesn't complain about his "desmadre," referring to his wild or chaotic lifestyle. Learning this word is a great step toward understanding real, everyday Spanish.
Ever tried to get over someone by finding their complete opposite? That's exactly what's happening in this song. The singer has found a new girlfriend who seems perfect on paper. She doesn't take forever with her makeup, she gets along with his friends, and she doesn't complain about his messes. Even his mom thinks she's perfect! He says he specifically looked for someone who had nothing in common with his ex, especially not her "mala actitud" [bad attitude].
But there's a huge catch. When things get intimate, he realizes the new girl is missing something important. He sings, "Pero le hago el amor y ahí si quisiera que se pareciera" [But I make love to her and that's when I wish she were more like you]. The new girlfriend is just too... nice. She's so good that she can't "hacerlo con maldad" [do it with wickedness]. So now he's stuck in a tough spot, asking himself how he's supposed to forget his ex when every time he's with his new partner, he's just "imaginándote a ti" [imagining you].