This is just because the current libpoppler-glib-dev depends on libgtk2.0-dev instead of libgtk3.0-dev. It uses GDK in its API, for instance. This would happen to people using GTK+ 3, not just gtkmm 3.
However, GDK is only used by include files when POPPLER_WITH_GDK is defined. POPPLER_WITH_GDK is apparently defined during the poppler build, and I don't think Ubuntu's libpoppler-glib was really built with that defined. So maybe the dependency is unnecessary. Reassigning to libpoppler-glib-dev for investigation there.
If GTK+ (or GDK) really is needed, it would be great if libpoppler was ported to GTK+ 3, but that doesn't seem to have even happened upstream yet.
This is just because the current libpoppler-glib-dev depends on libgtk2.0-dev instead of libgtk3.0-dev. It uses GDK in its API, for instance. This would happen to people using GTK+ 3, not just gtkmm 3.
However, GDK is only used by include files when POPPLER_WITH_GDK is defined. POPPLER_WITH_GDK is apparently defined during the poppler build, and I don't think Ubuntu's libpoppler-glib was really built with that defined. So maybe the dependency is unnecessary. Reassigning to libpoppler-glib-dev for investigation there.
If GTK+ (or GDK) really is needed, it would be great if libpoppler was ported to GTK+ 3, but that doesn't seem to have even happened upstream yet.