Is the sentence, "The grave's a fine and private place…" from Andrew Marvell's “To His Coy Mistress,” an example of overstatement?
No, that is really an understatement.
An "overstatement" is an exaggeration, or stating something too strongly. With the grave being the penultimate example of privacy and being cut-off from life itself, the phrase "a fine and private place" is really treating it more lightly - something of a 'coy' statement itself, relative to the subject of the poem.
The word "fine" is also softening the image - understatement - rather than reflecting either the harsh reality or making it more pronounced through an overstatement.