based on my limited understanding, i would imagine that women don't really want to date a man who ISN'T "nice", but rather that they want men who are assertive, confident, and mature and a lot guys who would simply describe themselves as "nice" don't fit the criteria (because really, you are probably kind of bland if that is the first adjective you'd describe yourself with)
it varies, some "nice guys" really are just nice guys who don't attract a lot of women because they don't have "swag", but a lot of "nice guys" just bitch about being friendzoned or whatever and they don't understand that women aren't really obligated to give them anything in return for it, they're self-centered and they just get more and more bitter because they don't understand that people can't help who they're attracted to, and for whatever reason a lot of the time that may not be you. it doesn't necessarily mean there's anything wrong with you or her, just that you don't elicit the same reaction from her. however, if you don't make your intentions clear and are just hanging around them in hopes of getting laid, you're not really a friend, and you're not very nice. there isn't really such a thing as a friendzone imo unless you let there be one, if she's not interested you can just be like "bye" and go do your thing. it's blunt but it's much more honest and better for both parties in the long run.
i'm probably talking out of my ass here (at least partially) because my experiences with women are pretty much null, i've never had a girlfriend or any sort of romantic experience whatsoever. the only girl who's ever been interested in me was someone i knew from the internet ' 3', and i don't do online dating so that didn't work out. i've had feelings for women before but due to poor self-esteem and/or social awkwardness i never really did anything about it. dunno if i'd classify myself as a "nice guy" in that sense. i used to get sulky and bitter when i thought the lack of reciprocation was obvious (not because I felt like I was entitled to anything from them or because I felt that they were somehow at fault, but just a needlessly negative and emotional response), but i really don't anymore because i know that it doesn't mean anything bad about me as an individual.