Are we here really comparing both things? Be single is not and never should be a requisite to be a idol, hold simple notes should be
But is not, there are visuals and 'main dancers' that are bad at singing, yet they become idols.
Sakura is who she is, she is not going to get better, telling her to go back to take singing lessons is bullying, no lessons are going to improve her.
She is an idol for her personality and her ability to connect with fans, being real and not taking shit is one of those personality traits that she has, not everyone is going to like her.
Shuhua from G-Idle has a similar thing going.