For description in this case, given you need to list and have unique descriptions for each package (or it defaults to the main one, which isn't applicable to the sub-packages), you'd want to do:

description   :
    - zathura is a highly customizable and functional document viewer based on the girara user interface library and several document libraries
    - mupdf : mupdf plugin for zathura
    - poppler : poppler plugin for zathura

Also I'd make sure the mupdf and poppler sub-packages depend on the zathura main package, whether it's linking and thus automatic or by adding zathura as a rundep for them.