There seems to be neither a keyboard shortcut, nor the possibility to add a button to the tool bar, to rotate the text 90° right or left. Always going through the menu, and even there not having the option to rotate 180°, seems like a feature missing.
You can actually both add it to the toolbar and add a shortcut. I do think though that having a shortcut by default could make sense.
A possibly relevant merge request was started @ https://invent.kde.org/graphics/okular/-/merge_requests/923