Calling all fashionistas. Clothes are a big deal in this play—just ask Cloten. Here's how Imogen compares him to Posthumus: "His mean'st garment / That ever hath but clipped his body is dearer / In my respect than all the hairs above thee" (2.3.152-154). When Imogen says this, Cloten goes crazy on her. He keeps repeating "garment" and is super insulted. Why?
Well, Cloten is all about appearances. Early on, we hear him boasting of his abilities to anyone who will listen—in fact, that's what gets him killed. Cloten thinks clothes are representative of someone's social class and worth. He can't believe Imogen thinks he's worse than Posthumus's cheapest digs. Posthumus is basically a commoner, while Cloten is a prince. How dare she?
Cloten's so convinced of his superior status that he borrows Posthumus's clothes to prove a point to Imogen. When he wears these borrowed clothes, he's convinced he looks stunning—"How fit his garments / serve me!" (4.1.2-3)—but he can't cloak what's inside those clothes: Cloten himself.
What Cloten doesn't get is that Imogen doesn't want him, no matter how he's dressed. He's a fool, and no garments will change that. Cloten, on the other hand, really believes that what you wear determines who you are.
We see the exact opposite idea when Posthumus picks out his own clothes. When he's supposed to fight for the Romans but actually wants to fight for the British, he changes his clothes to fit in. He proclaims, "I'll disrobe me / of these Italian weeds and suit myself / as does a Briton peasant" (5.1.22-24).
Posthumus knows the look doesn't matter: it's the man the counts. In fact, he (and other misfits) save the king's life and win the battle for Britain, all while wearing cheap, dirty rags. He proves that clothes don't make the man.