The truth is that there's usually a very boring, predictable, unadventurous, non-thrilling side to people who are 90% nice and 10% everything else. It's not that nice guys and nice girls are not liked, but that they can genuinely be outshined and upstaged by someone else. There can be something very appealing when it comes to unapologetically raw, unfiltered people who stop being nice in order to keep it real, get ish done, and express themselves.
In my experience as a nice girl, generally the only guys who are into "nice girls" are "bad boys." To corrupt them, take advantage of them, who knows. Shy, nice guys are usually more attracted to confident girls, as well as the confident nice guys.
Now there are exceptions to EVERY rule, so don't get worked up if your experiences are different. This is just what I've witnessed myself, being a former shy nice girl and later becoming a confident nice girl.
Yes, though it's more like: guys don't like girls that are too "innocent." It's basically standard for most people that girls be experienced sexually but not "too experienced", be girlfriend material but not too "pure" or "frigid", and be confident but not "too confident." Stuff like that.