Re-reading Frankenstein, it strikes me that many contemporary readers must see the style, language, and general writing as second-rate. The book was written with the conventions and language of Romanticism; it came before the Realism Revolution of the novel and well before the subsequent Modernist Revolution. The twentieth-century reader's tastes for what is good writing and good fiction were formed by realism and modernism; indeed, from the perspective of realism and modernism, the language of Romanticism is second-rate.
But that's the thing about tastes. The preference for the twentieth century style of writing (invented, some would say, but people like Flaubert, Twain, and James) is a preference, a taste. It doesn't necessarily mean that this is an inherently superior style of writing. A contemporary of Shelley would not have appreciated the novel written in the style of Flaubert (and indeed, if Shelley were born a hundred years later and written the same book, she may have written it entirely in the style of realism. Or not at all. But then, considering Frankenstein's impact, it's hard to really predict the history of literary tastes and moods without it).
In the 19th century, there was a critic who said that the slave narrative was the only authentic American literary genre; the implication was that the slave narrative was superior literature. This is the century that gave us Melville, Hawthorne, Twain, Poe, Emerson, Thoreau, Whitman, Dickenson, and other titans of American literature. These are now more respected names than the authors of any slave narratives. Tastses change. A literary critic of one generation cannot possibly predict what writers will be thought well of by future generations, or what writers will be forgotten in a century, or what books will be considered masterpieces to the ages. We don't know; tastes change too quickly.
So if you read a Gothic novel, don't assess it by the standards of what makes good writing today.
Indeed, sometimes tastes change to such a degree that writers forgotten for decades or centuries suddenly come back into vogue. T.S. Eliot brought back Donne and the metaphysics; that type of poetry fit the tastes of modernism. Post-modernist thinkers look backward to literature before realism/modernism to works like Don Quixote and Tristram Shandy to find earlier examples of post-modernism. Tastes evolve, styles evolve, and it is difficult to know what will last.