In Japan thousands of such "idols" and that was a long time ago.
Well yes nowadays anyone can build herself up as some kind of celeb, so there's no guarantee that the shtick would work when AV use the celeb status to sell porn.
Especially for us in outside (of Japan) world, say an olympic skier, how many can name 1/10 of all current and recent Japanese olympic skiers? So what if one of them do porn? If I'm not aware of her prior of AV, then she's not a celeb for me. She might be (somewhat? barely? very?) well known to Japanese public, but if she's not known to me then I get no special interest in such celeb-JAV.
So my interest in this genre is when such AV offers something special. So it's broadly 3 (slightly overlap) categories: (1) hot girl, (2) star quality (3) production value and role-play. (1) is obvious, we all like good looking girls in AV, so a celeb is likely to be hot, so the celeb aspect is just assurance or promise that she is hot. But of course it's never a guarantee. Minor J-pop group favors girls for dance skills so both face and figure are variable. Good examples are Shoko Takasaki, a gravue model is of course quality assurance for hot girl. (2) star quality, fashion sense, make-up (yes even make-up is often done by assistant, the same mark-up artist can get better result depending on the girl) mannerism, gesture, facial expression etc. Great example is Yua Mikami, she was never a star in J-pop, but in the low-standard AV world, her star quality is bursting. A counter example is Aimi Yoshikawa, she is very hot, amazing figure and actually quite pretty face too, but she just doesn't know how to smile for the camera and in general manipulate her face. She has great hotness but negative star quality. (3) no matter how legit the original celeb is celeb, often the AV would play up the idol background and dress her up or have her perform a little bit. Sometimes it's a fake press conference with fake reporters asking fake questions. All these role playing leading to the sex scene are quite entertaining. No one does this better than Yua Mikami, even when the vid isn't a J-pop idol role play, she insist on dialing up the drama and acting and costumes and production value. If you look at all her S1 vids, and compare to all S1 vids on average, Yua Mikami vids have at least double the production value (budget). E.g. many more dialog lines, more (bad) acting, more shots (e.g. in corridor, in different rooms) before settling in the room where the sex is to happen.