Just a guess, but I wonder if this could be because of the "point & click" nature of the app and the static UI, both on desktop and mobile.
In the case of the mobile app, I'd say there is no animations (which is not necessarily a bad thing depending on who you ask) and a lack of gesture commands compared to something like Gmail (I know Joplin is an open source project with a small team of dev, so it's not exactly a fair comparison). Things like sliding a note to the left to edit the tags, showing tags and last edit time from the note list.
In a similar way on desktop, everything needs a click, hovering with the mouse is barely used anywhere. For instance, there is no way to see the tags of a note without opening said note (another click).
Another thing, while rounded edges and curvy shapes seems to be the current norm, Joplin's design is very angular, almost no rounded edges anywhere, only straight lines.
A good example of this is the CSS table of content that was shared a while ago. There is animation, rounded edges, hovering and as a result, it feels modern and dynamic. I realize not everybody likes animations in their face while working, but probably some ideas are worth considering among those.
There's also the lack of colors (like colored tags and notebook for quick at-a-glance identification), but this is starting to sound like a rant requesting a dozen new features when it's not, just an observation compared to some other apps.