Oliver’s speech hinges on colorful fashion terminology. To a person uninitiated in that world, half the words he says might sound ridiculous. Peik Lin serves as emphasis or punctuation to Oliver’s over-the-top comments. She’s an emphatic character, whose statements are like an exclamation point, which is results in comedy. Notice here that she doesn’t speak in complete sentences. 

Also, it’s sometimes funny on the page when characters only speak in reactive noises. Obviously, Rachel says more than “Oh!” and “Ahh!” in Crazy Rich Asians, but these reactionary noises make this scene more hilarious.