I’m of the school of thought that “nice” doesn’t really mean anything. It’s like a place holder, you stick it in a sentence because you’re expected to say something, and nice works about as well as anything else.
If I were to make a list of all of Mr. Del’s good qualities, I doubt it would even occur to me to include “nice.” He’s generous, kind, considerate (all of which are “nice” qualities), which I would list right at the beginning, and I would probably make it down to about item #612 where I was really fishing for things to say, such as “doesn’t fart overly much” before I would think to describe him as “nice.” It’s just too bland, and the ways in which I think about Mr. Del are deeper than that.
Some posters have suggested the possibility that she was expecting more, romance-wise, from the evening. Another possibility is that she suspected that perhaps a pass was in the works, and she wanted to nip that in the bud by using the “you’re too nice” line.
Not an entirely admirable thing, but I can understand it. I’m not trying to generalize, but some times when a person says “you know, I like you but I don’t think there’s a romantic future here,” some people respond with “well why not?” That’s a hard question to answer, it might not even have an answer other than “I just don’t feel that way,” but “you’re too nice” slips out as an easy (easier, in the moment) way to frame the rejection. Yes, I think it would be better for all parties if people stopped saying that, I also think it would be better if people stopped asking “well why not?” (I realize the OP didn’t say he asked that, I’m just talking about the concept of “you’re too nice” in general, in many cases it might be a pre-emptive strike based on past experience.)
So in general, I think that for the question “do girls find guys being “nice” off-putting?” the answer depends upon whether the guy and girl are defining “nice” as the same thing. If I only thought of Mr. Del as nice, he wouldn’t be Mr. Del. I wouldn’t find it off-putting, but I wouldn’t find it on-putting either. (Hey, I made up a word!)
I’m also going to very cautiously throw another idea into the mix, based on the specifics of the OP. You mentioned that she has some sort of history, and sees herself as a girl of a particular type (a type that you wouldn’t want). I don’t even want to speculate on what that might entail, it’s obviously private. She may have some issues with self-esteem, if she views that “type” and her history as a negative experience. Since you know her, and Tir knows her, and the rest of us do not know her, you can probably decide, based on her reactions to other things in life and other relationships, if that might be the case. HOWEVER (and I say that quite loudly) that’s not advice for you to take her self-esteem on as a personal crusade. It’s possible that both things could be happening at once – she could have low self-esteem and also not be interested in you romantic way. Odds are that you could do more to support her with her self image (if that even is an issue) as a friend, and see if other romantic things develop at her own pace further down the road.