Gonna go against the grain here.
Most (not all) nice guys aren't the evil people they are made out to be. Some might be naive or too bluepill in how they view relationships, but that's no crime.
There's nothing wrong with being nice. If she rejects a guy (or friend-zones him) and call him a "nice guy," in that context, she simply doesn't find him good looking.
His personality is good enough to be her friend, and she may even WANT to be his friend, but for dating, she suddenly finds his "personality" flawed lol. It just sounds SO very, very disingenuous. So I think it's more of a looks issue than women let on.