If this were a soap opera, I would assume someone had dumped acid on Elle MacPherson's dress and that, despite the fact it was being slowly eaten, she wore it anyway just to spite the person who smote her with the beaker of poison. Because that's what you do on a soap opera -- you smite, you spite. Plus, it's way more believable than, say, the idea that a woman known as "The Body" would wear something that a) corrupts it into something unremarkable, and b) looks like it was designed by that evil goop from Spider-Man 3.
Maybe it got Valentino while he was sketching. Seriously, that thing is a mess.
On the up side, though, with that hairdo she could totally pass as Katherine Heigl's mother on Grey's Anatomy. Best update your resume, Elle.
