Like any other stereotype, only sometimes true. Being nice isn't a bad thing, but being TOO nice is. When you're overly nice and sympathetic,
you make it easy for people to
walk all over you and use you. You might go out of your way just to avoid hurting people's feelings, even if what you were about to do or say didn't really have the potential to hurt. If you apply these traits to a guy interested in a girl, chances are she might not even take him seriously.
Also a nice guy might think that all he has to do is be nice to girl and give her compliments and
voila, sex will fall out. If you are this guy, then yes, you will 99.99% finish last.
The nice guy is not
manipulative, instead he just has no idea how dating and the world works. If he were manipulative and trying to take advantage of her, he should be called the "manipulatively
nice guy" or something like that.
If the term "nice guy" only referred to guys who are respectful of women, the phrase "nice guys finish last" probably wouldn't exist.