I agree, it's kind of funny in their website claim they don't put "bells and whistles" in their UI yet it looks way cluttered compared to K-9/Thunderbird.
No doubt it can work better than the aforementioned but it'd be nice if their devs could be a bit humble and recognize its UI could get some love and it would be beneficial for FairEmail.
Been using K-9 for a time and tried Thunderbird - it feels kind of smoother and there are some visual overhauls but ended uninstalling it because it was hard for me to tell which mails were unread (and which ones not) in the main mail list while in dark mode. I guess I'm not the only one who felt that way and surely they will fix it but this also left me the sensation that there's not much sense in switching to Thunderbird if you're using K-9.