That’s a good point as well. I don’t know how well we can judge her singing abilities based on PGSM. They weren’t the greatest, but it didn’t seem like any of the girls received much training prior to their recording. Rika, Mew, and Alisa did well because they’d sung before and knew what they were doing, but Miyuu, Rina, Ayaka, and Keiko were a lot rougher in my opinion. I got the impression that they were rushing to get everything done and didn’t really give them much of a chance to work on that aspect of the show.
Also Miyuu’s voice may have improved… though it’s possibly telling that we have no evidence either way.
Now that you mention it, it’s kind of weird that none of the girls have done much musically except for Chieko, Alisa, and Natsumi Kiyoura. Ayaka did “Cross My Heart” for one of her DVDs, Rina’s sung in a couple of Very Berry live events (which were… not her best work), Mew had one single for NHK, and Rika did a group charity song, but other than that, nothing.
Anyway, I take your point; if Miyuu’s voice hasn’t improved, it certainly limits her options. It is very interesting how in Japan, it’s almost a requirement for you to be a “triple threat” (singing, acting, dancing), whereas that’s not expected elsewhere that I know of.