Edgar Allen Poe's "The Fall of the House of Usher" is an example of Gothic literature and should not be considered a parody of the genre. Admittedly, it can sometimes be difficult to understand the fine line between serious literature and parody, as some parodic works are so subtle that they almost seem serious to many readers. However, I think it's safe to say "Usher" is genuine Gothic literature through and through.
First of all, let's consider what Gothic literature actually is. Generally speaking, Gothic literature is characterized by elements of horror, events that are or at least appear to be supernatural (such as ghosts, demonic forces, etc.), and themes such as ruin, decay, and the psychological disintegration of the story's characters. Also, Gothic literature often involves medieval-like settings (such as crumbling castles), and often focuses on a once prominent family descending into chaos, madness, complete obliteration, or all of the above. Horace Walpole's The Castle of Otranto is often thought of as the first true Gothic novel, and there's no denying that the genre has had a profound influence on many authors since then.
Now, with this context in mind, let's consider "Usher." The story features a wealthy family riddled with incestuous relationships and facing decline and obliteration. The events take place on a crumbling family estate that seems to be haunted by some kind of evil presence. Additionally, the estate is reminiscent of a medieval castle, as it contains many vaults which were used as dungeons in the past. Finally, the story focuses on Roderick Usher, a man who appears to be going insane, and who buries his sister alive and is then killed by her during a terrifying storm. All in all, the short story has a crumbling, gloomy estate, supernatural elements, psychological disintegration, and deep, dark family secrets. It's definitely Gothic.
Still, the question remains: is it serious, or is it a parody? To answer this question, it helps to look at the tone of the story. As an example, take a look at the story's opening passage (taken from eNotes' excellent online version of the text):
During the whole of a dull, dark, and soundless day in the autumn of the year, when the clouds hung oppressively low in the heavens, I had been passing alone, on horseback, through a singularly dreary tract of country; and at length found myself, as the shades of the evening drew on, within view of the melancholy House of Usher. I know not how it was—but, with the first glimpse of the building, a sense of insufferable gloom pervaded my spirit... I looked upon the scene before me—upon the mere house, and the simple landscape features of the domain—upon the bleak walls—upon the vacant eye-like windows—upon a few rank sedges—and upon a few white trunks of decayed trees—with an utter depression of soul which I can compare to no earthly sensation more properly than to the after-dream of the reveller upon opium—the bitter lapse into everyday life—the hideous dropping off of the veil.
The tone in this opening passage is immediately gloomy, ominous, and dreadful. Observing the House of Usher and its environs, the narrator looks upon a scene of utter desolation and decay, and he compares the experience to the unpleasant aftereffects of drug use. In short, the tone here is quite serious, and it sets the stage for the tale's later horrors. If the story was a parody, it would involve at least some sense of mockery in the tone, and Poe's prose would not be so thoroughly gloomy. Indeed, there really isn't a light moment in the text, and the climax is truly terrifying. As such, I believe that "Usher" should be considered a true example of Gothic literature, rather than a parody of the genre.