How does Mary Shelley's style of writing help create horror in Frankenstein?

First and foremost, Mary Shelley's topic is horrific, so anything else she did with style was almost second to the topic.

This novel squarely fits the definition of Gothic Literature. Some of the traits of that genre are: a decaying setting or decaying morals (think of Victor's workroom AND morals here); omens or prophecies, for instance, we see plenty of foreshadowing by Shelley throughout (Victor says at one point that one of them will end up dead and the Rime of the Ancient Mariner references serve as prophecies); supernatural or superhuman beings (the monster); and even damsels in distress (Elizabeth AND Justine!). So clearly Shelley's style is very much a part of a larger genre meant to horrify.

Her specific wording also helps create a horrific atmosphere. For example, she uses the words "dark," bodies," death," and "daemon," repeatedly.