Well, obviously the way people dress/look doesn't have any effect on the music they make... that goes for bands of every country/culture but all bands still pay some attention to their 'image'.
In the west I think attention to 'image' is mainly a marketing tool to use all possible ways to attract fans to a band as well as the music itself. However with some bands (both western and Japanese), the 'image' also becomes part of the band's music in a kind of wider way, combining the two to create what is really more like a theatre performance than a concert. Malice Mizer's and Moi dix Mois' concerts are prime examples of this. Obviously when you go to see a band live, you're seeing a 'performance' rather than just hearing the music, so the visual aspects will have a bigger effect. From this perspective it makes good sense to me to invest effort into the visual side of things as well (although I would say that, being a visual artist and also into theatre, lol).
In short, in my view great music is the solid core and the most vital, important thing for any band... and ideally they would 'wrap' this, so to speak, in great visuals for their live performances, to make an amazing experience for the audience. But as regards things like their CD cases, photobooks, magazine appearances, merchandise and that... well, they are 'only' marketing tools. In some ways they help to bring the music-buyers closer to the 'all-over' experience of the concert-goers, by including that visual element, but it can't be the same and anyway is secondary to the actual sound that finds its way onto the CD.
As for why girls like them when they look like girls... one theory is that it's because they are somehow unthreatening... they are men, but not strong muscle-bound men with sweaty hairy armpits (lol)... they make useful sex-objects for girls because they never actually have to engage with/relate to them as 'real men'... girls can admire their hair, their makeup and their slim build in ways they would also admire about another girl, but it's less easy to express your admiration for another girl without *perhaps* 'fearing' being thought of as 'lesbian' (this is only a wild and random theory, btw!) but they can happily admire the girly looks of a man, with the added advantage they can make sexy jokes about him
but without much possibility they will ever actually have to hang out with said J-rocker and be exposed to his uncouth beer-drinking, smoking, sweaty, hairy, inelegant, smelly, macho, posing, lazing, irritable and unpleasant habits.