Yes, duller is a Scrabble word!

Adjective

1. Lacking in liveliness or animation

2. Emitting or reflecting very little light

3. Being or made softer or less loud or clear

4. So lacking in interest as to cause mental weariness

5. (of color) very low in saturation; highly diluted

6. Not keenly felt

7. Slow to learn or understand; lacking intellectual acuity

8. (of business) not active or brisk

9. Not having a sharp edge or point

10. Blunted in responsiveness or sensibility

11. Not clear and resonant; sounding as if striking with or against something relatively soft

12. Darkened with overcast