It would be great to save this kind of thing for the reviews in the App Store, but if you have specific spots that you think could be improved, you could tell the マイコーさん (the developer) in the Visual Issues Megathread because he appreciates feedback
While I have small changes I make from time to time, to be honest, it's not going to change a lot. I am a solo-dev (who isn't particularly strong at UI/UX), so I share the layout between renshuu (the app) and renshuu (the website). Additionally, I have probably a magnitude more content, features, and customization than any other language app out there, so all that has to go somewhere.
Ultimately, though, I have made the choice to have it this way because it allows me to let everyone use renshuu for free for an unlimited time, no ads, no selling data, and no AI. It's a choice that not everyone will agree with, but it works for me :)
I like the design too. And I love that the app doesn't take up too much storage space, given the sheer amount of information, tools and cute features it has. I'd rather have a nice minimalistic app that gets the job done than a fancy flashy behemoth that takes ages to load and eats away more and more storage with every update.
But I like the design.
You can also write a review if you think there's something wrong. 
