Although Holly actually mentions Tiffany's (and having breakfast there) just a few times in the short novel, we think her reference to it is a pretty important element of the text, and it tells us a lot about her character. It's true that Tiffany's is expensive and that the things in it are out of her reach (at least for the time being), but it's the idea of Tiffany's and the perfection that Holly associates with the store and the brand that makes her feel better when she's scared or sad or angry. It's the belief that only good things happen there that makes Tiffany's so appealing to her.
So why choose this as the title? Well, there are several possibilities. The title draws attention to Holly without having to name her specifically – she's the only one in the book who loves the store so much. And since the novel takes place during World War II, when people were forced to ration everything from food to gasoline, there's something really pleasant about the thought of eating a meal in the middle of an opulent, shiny store that seems untouched by all the crazy things that are happening otherwise. And the title sounds like one for a novel that would be about light and fluffy stuff, which it really isn't, which we think this makes for a pretty cool title. It can do so much and mean so much, all while sounding like it's pretty insignificant.