I am quite happy with the editor in the desktop versions. However, the mobile editors would be easier to use with some more functions. On the one hand I think of some kind of a toolbar above the keyboard. Like it is visible in the screenshots of the MDNotes App (IOS). For android I think a good app for comparison is Markor. MDNotes is to my knowledge not open source and Markor is written in Java. Both of them interact directly with the filesystem.
Ipad Mini Landscape
Iphone SE Portrait
A toolbar with quick camera/photos access could come in very handy as iOS15 supports OCR (Live Text, only supported in newer devices) in the camera/photos app. As Markor seems to have a toolbar too, I assume there is a way to implement something in all mobile versions.
Maybe there is also already a custom made keyboard around that makes it easier to type markdown on mobile? Though, in terms of style it would most likely look better if it is from Joplin itself.
On the other hand I am missing a faster navigation on tablets in landscape orientation (notebooks -> note -> open/edit). A fixed notes sidebar would improve usability on tablets in landscape orientation. For the portrait mode and on too small devices I don't think a change of behavior would be helpful. Maybe there were already considerations or there are reasons why this was not implemented.