While I was contemplating who my patron/matron deities should be, my best friend (who had also shown interest in Wicca but was more geared toward a more generalized, "eclectic pagan" path), staunchly declared hers to be Cthulhu, and bought a bust of him for her altar...Naturally, I asked her why she was dedicating herself to a "fictional" deity. She retorted that all gods were fictional, to the extent that they were made up by people, and that Cthulhu was no less valid than, say, Zeus. I was rather taken aback, but I could think of nothing else to say. Her choice seemed childish and more of a mockery than an honest dedication, but she seemed serious at the time. Presently, I've started to think about her statement. Since all of humanity originated in Africa, and took thousands of years to spread across the continents, obviously it took some time for the cultures and religions that exist today to emerge. Where was Zeus before there were Greeks? Can one say that he existed before he was worshiped, written about, and depicted in art? I'm personally not comfortable saying that ancient peoples "made up" their gods, since it implies that they are not real, but my friend's words still ring in the back of my mind.
First, there's lots of different opinions as to the nature of the gods. I have problems with people calling themselves Wiccan and then declaring that there are no gods, that what we call gods are just mental archetypes; to me, that's a magic system, not a religion. Why would you honor a being you know doesn't exist? However, I believe in the possibility of an infinite potential number of gods, and I see no reason why all of them have been given a name by some culture. In fact, while I address my gods by historical names, than previous believers. I've come to peace with that. So I have no objection to non-historical deity names. Whatever information has passed between you and your god is really not for me to question. However, working specifically from fiction gets a little more dodgy IMHO, since those characters were deliberately and consciously invented. Although I suppose you could believe the author was divinely (and probably unconsciously) inspired. There's also a group of people called Chaos Magicians who will literally use whatever they find works, and that includes references to fictional characters. However, that is by their own description a magical practice, not a religion. It's a totally different paradigm in which to work. Worshiping Cthulhu, however, particularly in a Wiccan context, smacks of general silliness. Cthulhu (as popularly understood) simply doesn't fit in the Wiccan view of the universe, in much the same way that Satan doesn't fit either. I suppose it's possible this person has a very complex theological view on the nature of Cthulhu (far beyond my passing knowledge of Lovecraft), but somehow I doubt it, especially if this person was in high school.
This is actually a question I, myself, had been contemplating recently. More specifically, I wonder about the actual metaphysical nature of our relationship with the Gods: does our perception of Them in some way affect Their nature?
In any case, more relevant to your question, while I think the reality of the Gods and our relationship can be debated round and round, I don't give much consideration to gods in works of fiction.
Yes, the argument can be made that our concept of the Gods had to start some where and the most mundane explanation would lay in the creation of the Gods by men. So it's not hard to draw comparisons to gods created by modern men in works of fiction.
However, I think there is a singular, important difference in the two phenomena. Fictional gods are often created by one or two people and for the purpose of entertainment. They are the intellectual property of a single, infinitesimal human being, who sat down and intended to flesh out the minute details of a godly character. They are the very definition of artificial even in the loosest anthropological sense.
If the ancient Gods can be said to be inventions of humanity, they most certainly were not created for the purposes of novel entertainment but to explain, typify, and embody the mysteries of the Universe. They were not created by a single man or woman nor by a handful of men or women but by generations of tribes, cities, and nations.
I think this, simply, is the greatest and most important difference.
In a Wiccan context, I believe worshiping a God or Goddess straight from fiction is nonsensical and silly. Gods and Goddesses are actual beings in the Wiccan view and as far as I know do not spring up at any time from a human"s imagination. Now, if one simply borrows the name from a fictional character that most closely resembles traits of the God or Goddess they've come to venerate, I see no problem with that. IMO, naming and labeling is a human characteristic that does not necessarily translate into the language of the Gods. I feel that the Gods could care less what we call them, as long as we attempt to identify with and understand their nature.
Now, in a simply pagan context I find it harder to decide if it makes sense to worship a figure out of fiction as a God. Outside the structure of a religion like Wicca and in the broad range of eclectic paganism, almost anything goes.
I usually try to stay away from defining what's paganism and what is not. You could tell me you like to eat macaroni with light bulbs and worship the Great Ashtray God while contemplating the mysteries of the universe makes you a pagan, and I'd probably nod my head and say, "That's nice."
Seriously, paganism is such a broad range of beliefs, practices, and world views that it's difficult to decide what is and is not paganism. Honestly, worshiping a fictional God just does not smack of paganism to me. It simply does not fit with how I perceive modern paganism. It seems more New Age-y or some weird branch of psychology.
Now, I'm not a Cthulhu worshipper but Lovecraft always appears to me to have had a very compelling way of describing the wholly other. The Call of Cthulhu is quite an interesting story. The description of the cult has this:
That cult would never die till the stars came right again, and the secret priests would take great Cthulhu from His tomb to revive His subjects and resume His rule of earth. The time would be easy to know, for then mankind would have became as the Great Old Ones; free and wild and beyond good and evil, with laws and morals thrown aside and all men shouting and killing and revelling in joy. Then the liberated Old Ones would teach them new ways to shout and kill and revel and enjoy themselves, and all the earth would flame with a holocaust of ecstasy and freedom. Meanwhile the cult, by appropriate rites, must keep alive the memory of those ancient ways and shadow forth the prophecy of their return.
With that in mind, I can see how someone who finds Dionysus to be too tame would want to worship Cthulhu.
I would have a hard time following a deity that I know to be fictional, but at the same time I have to consider the people who are inspired by fiction to make a connection with the Divine.
Fictional characters are part of the human creative consciousness, and more than one fictional character or fictional universe has taken on a life of it's own, far beyond what the original creator could have envisioned. Devotion to Cthulhu is unusual, and I'm not sure how Wiccan it is, but I guess if she is serious, and not being glib about it, then who am I to question the validity of her god?
All I know of Cthulhu is the somewhat satirical appearances in the 'Oh my Gods!' web cartoons - but I too find it hard to take seriously people venerating this faux-deity.
However, I am more sympathetic to the idea of theopoiesis in general. Why should the egyptians, babylonians, celts, greeks, romans, norse et al, each have their own culturally specific pantheons; yet pagans in our culture be expected to merely reinterpret or reinvent the gods of previous cultures?
I honour the greek Eurynome, as the Mother of us all, but what if there are other aspects of my life which I do not feel have a direct correspondence with any anciently known divinity?
Or is it true that there are no aspects of our lives that are so innovative that there is no ancient deity that is sufficiently applicable?