This one happened during a period when moe was more brain-dead than cute, and the sister character who is supposed to be his motivation for everything just ruins the whole show for me she acts so stupid.
There's definitely a bit of untasteful moe stuff but it's not a big part of the show. But yea it would be better without it.
Mayushii acting stupid I cannot recognise at all. She's just extremely genuine and she's probably way more emotionally intelligent than any other character in the show.
It's been a very long time since I watched it, but I distinctly recall being very annoyed by her character. I just checked out the Wiki, and the very first line on her personality is "[she] possesses an extremely childish and naïve attitude for her age." While not stupid exactly , is just about what I recall hating about her.