Wheatley made use of the word "crown'd" to suggest that the king deserves to be bestowed upon with all the goodness. This word was used in his work "To the King's Most Excellent Majesty" in the following quotation: "May George, beloved by all the nations round, Live with heav'ns choicest constant blessings crown'd!"
He used the word "crown'd" metaphorically to convey that all the "constant blessings" should also be crowned upon him - much like crowning a king meant bestowing him with the power and responsibility of that role.