I would be willing to concede that if she is not heartless, she is entirely self- absorbed. As part of the Jazz Age social setting, Daisy reveals herself to be more concerned with a good party than the affections of another. The fact that she won't bring herself to leave her husband who is obviously completely ineffective in being any kind of emotional support reveals itself to be a selfish action as she realizes that his money is their ticket to the best party. She lets Gatsby do whatever and spend whatever in what she knows is a fruitless attempt to win her over. In the end, her inability to speak out for what is right sets in motion the events that prove to be Gatsby's undoing, while she is in search for that next party or piece of salacious gossip.
You can look at Daisy as heartless in The Great Gatsby, but there's another way to view her, also.
Daisy is a woman in a patriarchal society, a society dominated by men. For centuries in Europe and then in America, women who were not born into wealth had virtually one way of securing a better economic life: marrying a wealthy man. Daisy was not lucky enough to be born wealthy, and, in fact, for most of Daisy's life women in America could not even vote. Marrying a wealthy husband is her only hope of economically improving her situation.
Daisy certainly realizes this and behaves accordingly. She tells how she felt when her baby was born and the baby was a girl. Daisy despairs because her daughter will face the same obstacles that she has. Her daughter's only hope is to be a pretty little fool. That's her only option, just as it was for Daisy. That's what Daisy has done.
I'd suggest that to outrightly condemn her is to fail to understand the human condition, and particularly to fail to understand the plight of women in Daisy's time.
I would have to say that she is, at the very least, completely selfish. After all, think about all the things she does in this book.
She seems to care only about money. When she is a girl, she is in love with Gatsby but she breaks his heart when it is clear that he has no money. So she is willing to take money over love.
Then she seems to be falling for him again, to some extent, when she finds out he's rich. But then when she finds out he made the money the "wrong" way, she no longer cares for him. She is willing to kill Myrtle and to let Gatsby take the blame.
Sounds pretty heartless to me.