Sweetheart could very much just be like a teasing name. It does not necessarily mean she wants to date you, but it's definitely flirty and playful. I don't use that word often, but I'm not English speaking at home so makes sense, but if I were to speak English every day then I suppose I'd use that for my close friends, and boyfriend. I throw in random nicknames all the time.
Personally, I call a guy sweetheart only if I feel comfortable around them; 'sweetheart' is more intimate than words such as 'cute' or 'hot.' I don't think I'd call a friendzoned guy 'sweetheart.' I'm more likely to call them words such as "dude" or "buddy." It also depends--does she call a lot of people sweetheart?
That could mean anything, I call a lot of my guy friends "babe", "baby", "hun", not because I want to date them but because that is just what I do. It all depends though, if you really want to know, see if she calls anyone else by that pet name and then you'll have your answer!