"Ozymandias" is obsessed with transience; the very fact that the statue is a "colossal wreck" (13) says loudly and clearly that some things just don't last forever. But the poem isn't just about how really big statues eventually succumb to the ravages of time; the statue is a symbol of Ozymandias's ambition, pride, and absolute power, and thus the poem also implies that kingdoms and political regimes will eventually crumble, leaving no trace of their existence except, perhaps, pathetic statues that no longer even have torsos.
Even though the poem is obsessed with transience and impermanence, it also suggests that a work of art, however fragmentary, leaves a record of what has passed away.