I don't think you're over-generalizing... I've seen it happen first hand. Of all the friends I have - its the good-looking ones that get all the girls. They all have a wide range of personalities, but it comes down to looks. Nice, mean, selfish, caring - it doesn't matter. The girl will just complain and try to change them if they're unattractive in their personality - while still getting involved with them.
What's interesting is that both groups do the EXACT same things, but only the good-looking ones make significant progress. Girls say I'm nice, sweet, too good to be true, a sweetheart, the nicest person they've met, any woman would be lucky to have me, etc. The same lines all these "nice guys" hear before they get rejected. The difference is, sexual advances are always made on their end. Being nice doesn't turn any of them off. They LOVE it. Always smiling, flirting, touching. Hell, I am sometimes really shy, but that doesn't turn them off, either. I never get the friends line, or told I'm too nice.
Being too nice was never an issue. It was always being unexciting, acting like a chump, and so forth. No one was ever turned off by my niceness - even back in the day.
Another "nice" friend, however, gets rejected constantly. All of the girls want to be his "friend", and say he is nice. He never gets a chance. There is nothing we do differently. I act just as friendly as he does. Girls feed him the same lines - only, he gets rejected afterwards. Whereas, I usually get some kind of sexual advance. Doing the EXACT same things. The only difference is the looks. The same girls that reject him flirt with me, and others.
This applies to the jerks, as well. The unattractive ones hardly get any women, but the good-looking ones never have a shortage of them. It's the opposite side of the same coin with nice guys. Girls will say he has a bad personality, he's difficult, he annoys them, etc, to the unattractive ones...while adding "but..." to the good-looking ones. Or, simply ignoring all the bad, because they're "hot". Either way, they are given way more chances, and allowed to get away with much more. Or, just getting a chance at all.
I've seen and experienced this for as long as I can remember. Different groups throughout the years... All the same results.
I once asked a girl I dated about a comment a friend made about what not to do. He said don't be too nice to her, because you'll just be her friend. She told me straight up that he only failed because he wasn't physically attractive. He didn't make it past the first date. I bombed pretty hard, and still made it much further.