The poem shows the admiration and respect towards the King. For the word crown'd, it is the same theme that is discussed throughout the poem. The word exhibits the greatness of the King by showing that he deserves respect for his deeds.
Wheatley uses the word to express the idea that the king deserves to always be blessed with good things and with goodness. It's like a pun because the crown is also the king metaphorically and the crown is also a real thing wore by the king and it is also a metaphor for something that is bestowed upon you, in this case good things.
