Depends on the situation. Do you know her well? Did you just meet? Did you mean online? What kind of current relationship do you have with her?
Although usually when people put in the effort to write a letter, they are interested in you one way or another. It may not necessarily be for dating although it can be. So you know that you have caught her eye.
Closing a letter with xo and her name may not actually be such a big deal. It can be if the girl is shy or doesn't really use that kind of notation often (the xo part), meaning that she probably does like you. But if she's that kind of girl who does that will all her friends, then it's probably just part of her signature and shouldn't be thought out as a sign necessarily.
But to analyze whether she actually wants to date you or not, I'd need more information. This is just what I've gathered from the information you've given me. Hope it helps a little.