Let not Ambition mock their useful toil, Their homely joys, and destiny obscure; Nor Grandeur hear with a disdainful smile The short and simple annals of the poor.
More figurative language, y'all! The speaker personifies Ambition and Grandeur in these lines. You can tell because (a) he capitalizes them, as though they were proper nouns or names, and also because he says that they're doing stuff ("mocking" and "hearing") that only people do.
So, what's the deal with that personification? The speaker is telling the readers that they shouldn't mock the hard work, or the homely, simple pleasures, or the unsung, "obscure" destinies of the poor farmers in the graveyard. But he doesn't come out and tell the readers to lay off the mockery—instead, he says that they shouldn't allow "Ambition" to mock them. He's sort of displacing the blame. Regular people wouldn't mock these honest guys—only Ambition would be that cruel. Maybe he doesn't want the readers to feel as though he's shaking a finger at them, even though he kind of is.
Same deal with the second two lines of the stanza: the speaker says that we shouldn't allow "Grandeur," or high social status, to smile disdainfully or scornfully at the day-to-day accounts ("annals") of poor people.
Again, though, it seems like the speaker is personifying "Grandeur" to take the edge off of this stanza so that it won't sound like he's scolding the readers.
(Rule Number 1 of Writing: If you want to earn money from your writing, you probably shouldn't attack the audience or make them feel bad about themselves.)