If you don't mind me asking, how come Mio is not really a popular character ?

Don’t misunderstand, Mio is a very popular character. Heck, she’s one of the three mascots. That said, compared to the other New Generations members, Mio can’t compete. I don’t know too much about pre-anime, but the anime did a number on her popularity. Both western and Japanese viewers thought she was being selfish and immature in episodes 6 and 7 (I disagree), and she hasn’t been able to recover since. It’s a shame really, Mio is great!