I guess that your point is that by the standard convention of writing, he is a poor writer? To be honest, I am not sure I disagree with that conclusion, but rather, it's logical assumption. I don't agree with a standard of "good writer."
Ideas alone do not confer a strength given to a writer, but the arrogance of Poe's work certainly do not diminish the strong nature of his work. I feel that the best means to explain my critique of your criticism is through a similar situation with poetry.
Personally, I think that poetry lends itself more toward strict principles than prose. However, beside the most perfect and precise Emily Dickinson sits the imperfect and yet utterly loved Walt Whitman. In almost every sense, I find Dickinson superior, but the world should not surely agree.
As for Poe, he does not enjoy the sheer brilliance of authors I personally hold more dear, such as Orwell, Gaines, and Faulkner, but to consider him merely average is akin to saying Paul Morphy is a poor chess player. The capability is entirely different and demonstrates a genius all its own, and the statement strikes my senses as somehow offensive due to its inaccuracy. Poe, I suspect, is carried by having some writing skill combined with insight into American Goth that did, undeniably, affect the future of literature including Sir Conan and Flannery O'Connor. To that end, I still hold him a great writer, just in a different sense.