I have to agree w Xenophobia. Nowadays, the word "nice" is a cop-out, a default response, a word you use to describe something you consider "blah", but don't want to be rude to that person. When a females describes me as "nice", to be honest I get slightly offended. Why? Because I think, "You couldn't use another term to describe me? Nice is so blah." I'd rather be called "sweet" than "nice" lol.
I think this girl thinks that, by you saying she's "nice", that she "doesn't strike your fancy", & therefore you're uninterested.
This is why I try to avoid using the word "nice". It's connotation is all screwed up nowadays, & leads to mis-communication. Tell this girl she's "cute", "sweet", "adorable" or "bubbly"...all are better than "nice".