Somewhat ironically, and for better or worse, macOS Big Sur has changed the norms for application icons to more closely resemble those of iOS:
That said, the corner radius is only one of the aspects of the icon design that doesn't follow the (updated and ever-changing) HIG. Icons are generally supposed to at least vaguely resemble physical objects while still implying a rounded rectangle. Might it be reasonable, say, for the Joplin icon to have a binding on the left-hand side in order to suggest the appearance of a notebook?
I could try and take a stab at this myself...