The thing about Uzuki's rise in popularity is the fact that she resembles so much like Haruka, people assumed that she's the Haruka of Cinderella Girls (to which I believe as well). This helped on setting up her eventual popularity. But her popularity was mainly based on the fans poking on the fact that she's left in the dark.
Why is Uzuki's being left in the dark noteworthy? Because she's "the Haruka of Cinderella Girls".
Sadly, I can't say the exact thing for Mio. I'm a Mio fan, but I have to be honest: She resembles no one from 765- Uzuki's like Haruka, Rin's like Chihaya. Mio shares the same zodiac and blood type as Miki, but the similarities may have ended there, and the fans may not be aware of that or just didn't see it as important.
Still, I say that Mio can be brought into the spotlight. But as I probably have said in the past, it's fan interaction that's gonna determine that. Something I'd do if I had a working computer around.