In my opinion, the statement to which you refer (which can be found in the first chapter of The Great Gatsby), reveals quite a lot about Daisy's character.  Before delving further, let's look at the comment in context:

"I love to see you at my table, Nick.  You remind me of a--rose, an absolute rose.  Doesn't he?"  She turned to Miss Baker for confirmation: "An absolute rose?"

This was untrue.  I am not even faintly like a rose.  She was only extemporizing, but a stirring warmth to you concealed in one of those breathless, thrilling words.  Then suddenly she threw her napkin on the table and excused herself and went into the house. (15)

Now, I'm sure Daisy's "rose" statement is open to interpretation, but in my opinion, it shows what a shallow ditz of a gilded flapper Daisy really is.  It proves that Daisy (as Gatsby will later confirm) is the voice of inherited money, . . . and not much else.  It's also interesting that Nick uses the word "extemporize" to describe Daisy's words.  Extemporize, of course, means to do something, particularly to perform or speak without prior planning or thought.  Yeah.  That would be Daisy.  This is the gal who lost it over Gatsby's shirts.  Daisy's character is weak and, as a result, most of her statements are weak as well.  In fact, even when she tries to be serious, she falters.  She tries to convince Tom that she's leaving him by saying, "I am, though."  Sad, isn't it? 

Thus, I suppose it started with Daisy's "rose" comment when I say that I feel sorry for her.  There is no better evidence of how money can corrupt a mind than the character of Daisy Buchanan.

The significance of Daisy's statement is that, in one respect, it implies the superficiality of her character. It is a silly thing for her to say, and she says it for effect, to seem warm and charming. Nick knows he is "not even faintly like a rose," but he recognizes the emotional effect Daisy can create with such a ridiculous statement:

She was only extemporizing but a stirring warmth flowed from her as if her heart was trying to come out to you concealed in one of those breathless, thrilling words.

The key element of Nick's description of Daisy is his use of "as if." Daisy's heart was not reaching out to Nick; she was emotionally dishonest and manipulative.

Daisy's words to Nick, in the context of the chapter, suggest another interpretation. She calls Nick "an absolute rose" immediately after Tom leaves the table after the butler gives him a message. Daisy knows, and Nick soon finds out, that it is Tom's mistress who has interrupted their dinner. Daisy's comment to Nick helps her cover Tom's awkward, sudden departure; it also represents an effort on Daisy's part to make herself feel less alone, less powerful, and more valued in someone eyes, even Nick's.

Why does Daisy compare Nick to a rose, and what is the significance of this statement?

Without doubt, Daisy Buchanan is affected and uses phrases that are exaggerated for the situation. She enters the narrative with charm, beauty, and effusive delight. One of her favorite ways of flattering people is to call them "a rose." In Chapter One, for example, she personifies what Nick describes as the "young breath-giving air" of West Egg. 

When Nick arrives at the Buchanans' home, the supercilious Tom shows Nick his "nice place here," sweeping his arm at a sunken Italian garden and a "half-acre of pungent roses." Then they walk through a hallway "into a bright rosy-colored space..." and across rose rugs. Daisy's voice compels people to listen to it as it charms them. After the butler tells Tom he has a phone call (which the reader later learns is from his mistress), the effusive Daisy tells Nick how delighted she is to have him for dinner:

"I love to see you at my table, Nick. You remind me of a --of a rose, an absolute rose. Doesn't he?" She turned to Miss Baker for confirmation. "An absolute rose."

Although Nick realizes that Daisy is affected in her speech, he narrates,

She was only extemporizing but a stirring warmth flowed from her as if her heart was trying to come out to you concealed in one of those breathless, thrilling words.

Daisy becomes part of the ambiance of the room, the voice of the breeze that blows through the rosy room, the scent of wealth and delight and the promise of romance. Daisy herself is a romantic illusion, emotionally dishonest and shallow. There is no "stirring warmth"-- only "thrilling words" as Daisy extemporizes.