Quem dera is an idiomatic expression in Portuguese that translates to "If only" or "I wish." It expresses a strong desire or a longing for something that is not currently true or is unlikely to happen.
In "Coisa Mais Linda," Caetano Veloso uses this phrase to express a deep wish that the beauty of a flower's spring (primavera da flor) could possess the same enchanting aroma and beauty as the woman he is singing about. It adds a touch of poetic yearning to the lyrics, making the compliment even more profound.
Caetano Veloso opens with a bold claim — “Coisa mais bonita é você” (“The most beautiful thing is you”) — then spends the rest of the song trying to prove it. He can't quite find the words, “Eu não sei porque você” (“I don't know why you”), so he reaches for the biggest comparison nature offers: “Você é mais bonita que a flor” (“You're more beautiful than the flower”), wishing the flower's whole spring bloom, “Todo esse aroma de beleza que é o amor” (“all this aroma of beauty that is love”), could somehow measure up to her.
By the second half the song flips its own logic: next to her, nothing else even counts as real. “Não existe a flor” (“The flower doesn't exist”), “Nem mesmo a cor não existe” (“Not even the color exists”), “Nem mesmo o amor existe” (“Not even love exists”) — a soft bossa nova way of saying she's made every other kind of beauty beside the point.