Was Mugi actually way smarter than Mio, but let Mio get to be the "smart one" in the group just to help boost her self-confidence? I mean keyboardists kinda have a reputation of being the really smart nerds of any band. They tend to know the music theory a lot more, and she's been playing since she was 4 years old. She probably comes from excellent breeding, her father is an important business leader and could have found a top quality wife with superior genes. Plus Mugi is an alien who has come to our planet only to observe the creatures here, not interfere in the natural development of our species. I always got the impression she was holding back her talents a bit, downplaying them so that Mio could shine more brightly, and Mugi simply manipulated the band from the shadows to help them achieve their successes without receiving much credit for herself.
No hate towards Mio, who is genuinely really cool and sweet and deserving of her fan club. I love Mio. But Mugi is actually the superior form of life, is she not? And as a superior being, she also has superior empathy and kindness, and knows that what Mio really needs is to feel, in a safe way, that she is deserving of the attention she receives for abilities and traits. If Mugi were to show her true power level, Mio would always feel like second fiddle, and remain in her shell. Mugi is the ultimate support character, doing what Ritsu does for Mio, but in a more subtle way so that Mio never thinks Mugi is patronizing her or pushing her too forcefully. Mugi even sits out of Mio's line of sight so that Mio won't be constantly comparing herself to Mugi, and instead is always looking at the two retards across from her, and their short kouhai.
Truly Mugi is a gentle, warm soul who is only thinking of what is best for others with her superior intellect, helping Mio to grow into a confident young woman, plying Yui with enough sugar so that her one brain cell can work harder, and making Ritsu feel better about being a poor.