Yes, mule is a Scrabble word!
...and is worth 6 points in Scrabble, and 9 points in Words with Friends
mule
Noun
1. Hybrid offspring of a male donkey and a female horse; usually sterile
2. A slipper that has no fitting around the heel
What Scrabble words can I make with the letters in mule?
3 Letter Words
2 Letter Words