Yes, sumo is a Scrabble word!
...and is worth 6 points in Scrabble, and 8 points in Words with Friends
sumo
Noun
1. A Japanese form of wrestling; you lose if you are forced out of a small ring or if any part of your body (other than your feet) touches the ground
What Scrabble words can I make with the letters in sumo?
3 Letter Words
2 Letter Words