Super Paper Mario is a love story; it is a story that is about love. From the grand story of the Star-Crossed Lovers that drives the plot to the NPC in Flipside pining away for her lover who's addicted to gambling, nearly every character in the game discusses or shows love. The boss battles in the game are, with a few exceptions, symbols of love being corrupted in some fashion.
The Wedding
The game begins with the forced marriage of Bowser and Princess Peach, conducted by Count Bleck. While marriage is usually a pure and beautiful ceremony, an act of creation, this wedding is used only to begin the destruction of the universe. Neither of these characters truly love each other, (though it has been heavily implied that Bowser legitimately likes Peach, but not to the point of love, and it's definitely not true the other way around) so it becomes an abhorrent act. Conversely, Blumiere and Timpani love each other dearly, and their love saves the universe.Francis
Francis is probably the most subtle example of a boss representing corrupted love. It's clear as you play through Bitlands that he's very lonely, and has directed his affection towards physical things (like anime and dolls) in the place of real people (and, as one of the questions to get into his room indicates, has lost the few friends he has over these physical things). When he finds Tippi, he's so lonely he captures the talking butterfly and treats it exactly like he would any of the other physical things he has. When Peach shows up, he mistakes the attraction he has to her for full-on romantic love and tries to treat her (once again) as one of his physical things, in this case, a Visual Novel. In the end, none of his physical things make him any happier, and Peach beats the crap out of him.Note that you can meet an ex-toy collector in Flipside - he says he gave up all that when he realized it didn't make him happy and ended up getting married. He also still owns some of the cute stuff - this reinforces the idea that you can have fun things like that, and still be sane.
The Pure Hearts
As a device to drive the plot, it's suitable for Mario to be searching for "pure hearts."However, this takes a more interesting approach when considering Chapter 7. At the end of the chapter, you learn that Luvbi, the Nimbi you've been escorting to the top, is actually the 8th and final Pure Heart in the game. She was turned into a Pure Heart by her parents, who only ever really wanted a child of their own, or a daughter to be exact. Though she was created in the form to protect the Pure Heart, it ultimately tore them up inside to have them lie to her about who she was or what her purpose was for the world, thus symbolizing parental love towards a child. Her being turned into a Pure Heart once more ultimately symbolizes this fitting phrase: "If you truly love someone, you have to let them go."
