Yes is the answer. Many mortals will say “not funny” and that is correct. “Unfunny” is simply another way to say “not funny”. Remember the “golden rule”: the fewer words the better. One word is better than two words.
Here are some examples using “unfunny”.
* It is an unfunny political joke (… un broma política sin gracia).
* Mr. Sánchez told an unfunny joke (…sin pizca de gracia).
* Mr. Sanchez´s jokes are painfully unfunny.
* The Spanish cabinet (cabinete) is full of unfunny clowns (…lleno de payasos sin gracia).
So, have you heard or told unfunny jokes?