Strictly speaking, that is. For to say that God is full of mercy is to imply that mercy exists outside of Him. God cannot be full of anything because God is everything. He can have no attributes because attributes imply deficiencies and imperfections, neither of which exist in God.
Technically, God is not merciful, but He is infinite mercy. God is not full of justice, but He is justice to an infinite degree. God cannot be full of truth and goodness; God is truth and goodness.
These things are true, and yet God is still referred to as merciful. In fact, God Himself tells us that He is merciful. Why? Because the spirit of the law is more important than the letter of the law. God understands how cumbersome it would be for our poor human intellects to make this slight distinction, and He knows it is not necessary to do so. The point is that God deals with His creatures in a merciful manner, so for all intents and purposes, He is merciful.
Still, it’s interesting to note the distinction. Perhaps this truth will lead us to a greater reverence of and respect for God. At the end of the day, however, we still say the sun rises even though we know it doesn’t. So let us say that God is merciful, even though we know He isn’t. God is mercy, and the important thing to remember is that His mercy is above all His works.