Yes, her looks do sell albums. But not for the reason you describe.
Britney Spears sells albums primarily to pre-teen and teenaged girls. While part of the reason they buy her albums is that it’s poppy, well-produced, upbeat music, there’s a heavy marketing element involved. To really sum it up, little girls buy her albums because they look up to her. Britney Spears is, to a little girl, what THEY want to be - sexy, pretty, popular, desired by boys, and powerful in a feminine way. For a popular music fan - not just Britney, but all acts - being a fan is about more than music, it’s about liking the musician and vicariously enjoying their power. So musicians that sell to different markets will market themselves as different kinds of role models:
- Pop princesses, like the ones you names, market themselves as ideals of sexiness and beauty.
- Heavy metal bands market themselves as tough and mean guys who could whip your ass in a bar fight, even if they’re 5’3" geeks.
- Goth and punk bands market themselves as chatoic, out-of control weirdos who (most importantly) will scare parents.
- Country and western artists market themselves as aw-shucks truck-drivin’ patriots who ain’t much fer book larnin’.
- Progressive rock bands market themselves as cutting-edge fashion plates who are ironic and disaffected and might live in big loft apartments.
So to that end Britney markets herself in an image that little girls will look up to and idolize. It is perfectly normal that little girls will idolize a beautiful woman over a plain one, because women just do that naturally. (Men do, too - men will respect another man FAR more if that man is tall.)
Britney will naturally sell more albums if little girls idolize her; if she’s a role model they will buy her albums in large part for that reason alone. Then they can sing and dance along and fantasize that it’s THEM who’s onstage before 25,000 shrieking fans, who can wear any clothes they want, get any boys they want, and be the Most Popular Girl In School. They won’t necessarily consciously think that; ask a Britney fan and she will say Britney is “cool” and assert that Britney is, in fact, a really nice person and deserves all her success. If Britney was fat and had warts, they would find her less cool.