The "pathetic fallacy" is a term coined by John Ruskin to describe what he considered misuse of personification, or treating animals or elements of nature as though they were possessed of human intellect or emotion. Although some of the metaphors used to describe the weather in Sonnet 14 involve a limited amount of personification, they don't really fall under the category of pathetic fallacy as it is normally defined by literary critics, because they do not attribute emotions or intellect to nature.
For example, when Shakespeare refers to the sun as "the eye of heaven", he doesn't actually humanize the sun by making it look at anything or respond intellectually or emotionally to anything; instead, we get a sense of both the importance of the sun and its ability to reveal the world by giving light.
The line "Thou art more lovely and more temperate", although again moving close to pathetic fallacy, in the notion of moderation, does not cross all the way into fallacious territory, as in fact "temperate" can refer to moderate weather as well as human temperaments.
The metaphor of "his gold complexion dimm'd" is not an example of a pathetic fallacy as it is a purely physical description with no emotional or "pathetic" component.
The main literary device used in Sonnet 18 is metaphor. It also uses rhyme, meter, comparison, hyperbole, litotes, and repetition.
For more analysis of Sonnet 18, check out this video:
In Sonnet 18, the speaker describes his lover's beauty and all the ways in which their beauty is actually preferable to that of a summer's day. One way that they surpasses a summer day is that their beauty is more temperate, more even, as "Sometime too hot the eye of heaven shines, / And often is his gold complexion dimm'd" (lines 5-6). This line characterizes the sun as heaven's eye -- a kind of personification of heaven (as possessing an eye) -- however, this does not qualify as an example of pathetic fallacy because no real emotional or intellectual content is ascribed to the use of this device.
The speaker also tells his love that "thy eternal summer shall not fade," by which he means their youth, the most lovely season or time of their life (9). Neither does this metaphor qualify as pathetic fallacy, for the same reason as above.
The speaker also personifies death when he says, "Nor shall Death brag thou wander'st in his shade" (11). In this figure, the speaker ascribes to Death the ability to boast, and this personification does add to the emotional and intellectual content of the poem; therefore, it is an example of pathetic fallacy. The speaker means that Death will never be able to brag that he possesses the beautiful lover because they have been immortalized -- freed from death in a way -- by the speaker's words in this very poem.