"Weird but in r&b, rap, hip hop, it seems looks does not matter,"
Disagree. Image is very important in these types of music too, there are some very good looking r'n'b females on the scene, can't speak for the men. There's a difference between pretty/perfect (pop) and 'rugged' (rap/rock), both can be attractive.
I think kids into pop tend to be put off by artists who look slighlty 'dangerous' and more comfortable with wholesome looking artists. Different product aimed at various markets, that's what it all boils down to.
Think about when you are a kid how scarey very old people look to you, you feel more at home with people who look young and like they might be into the same things as you. When you get a little older into the rebellion stage then you look for the rebellious non-conventional types of artist (image). When you get older still it goes full circle and you are put off by someone who is trying too hard to promote a certain type of image and you look to the product more than the package - so older people get into jazz etc, where image is not so important just the music.