I feel like there's multiple things of importance here...
#1: CG songs weren't vocal-heavy (at least from what I can tell), and if there were such lines they usually went to Momoko/Funakki/Yamaki
#2: Ju=Ju does usually improve the vocals of their newly added members. But she's had some decent solos before then (such as her Koi wa Magnet after her health hiatus)
#3: Manaka does sound really good here, and I loved how she sounded on the chorus of this... but there was a song she had either this birthday event or last year that was just painful to listen to as she was completely off key or something. So idk if she's consistently good, or just on these one-offs.