Yes, dumb is a Scrabble word!
...and is worth 9 points in Scrabble, and 12 points in Words with Friends
dumb
Adjective
1. Slow to learn or understand; lacking intellectual acuity
2. Temporarily incapable of speaking
3. Lacking the power of human speech
4. Unable to speak because of hereditary deafness
What Scrabble words can I make with the letters in dumb?
3 Letter Words
2 Letter Words