A theme of Multiversity is the power that fiction has, and that ideas have. The Gentry represent bad, dark, horrific, nasty, hateful, etc ideas, and they can only get in your head if you let them in. Morrison said in an interview something along the lines of "nobody is careful about the ideas that we allow into our minds. Everything we read, we're just absorbing these thoughts and feelings without thinking about the effects on our mind." That's what the idea of the Gentry are.
Ultra Comics stems from that idea. First the question is asked "How can a superhero affect our reality?", and drawing from the same place as the Gentry, the answer is "through our thoughts". By reading about heroes, our lives are affected. Sometimes they simply brighten up our day, or engage our imaginations, sometimes they can drive us to commit good deeds and even save some people from suicide. Therefore, in a way, a comic IS a superhero, taken form in our world.
Ultra Comics is literally a comic book, with the entire way he is born and formed in the story representing how a comic book is printed and written.
The other side of Ultra is Empty Hand. A story needs conflict, and by reading and consuming the story you are creating the need for that conflict. The story must go on, with the histories and nature of the comics becoming more complex and dark as they continue indefinitely.
Ultra Comics is really the only issue that's meta to that level, and that's because it's the issue about how superheroes exist on our Earth. The rest of the stories are simply analyses of different types of comics. You've got a pulpy action story, you've got a whimsical Golden Age power fantasy, you've got a slow and methodical and political take on superheroes, you've got young legacy characters more focused on drama than fighting villains, you've got a dark and complex alternate history story, and so on.
