Yes, mutt is a Scrabble word!
...and is worth 6 points in Scrabble, and 8 points in Words with Friends
mutt
Noun
1. An inferior dog or one of mixed breed
What Scrabble words can I make with the letters in mutt?
3 Letter Words
2 Letter Words
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Words that can be created with an extra letter added to mutt:
S:mutts