Yes, fatter is in the scrabble dictionary.

Is it fatter or fat?

adjectivefatcomparativefattersuperlativefattest

How do you use the word fatter?

1. And he grew fatter every day.
2. There are plenty of fatter fowl to pluck.
3. A fatter creature …
4. And the baby grows gloriously fatter in spite of everything.
5. Have ready some very thin slices of cold boiled ham, the fatter the better.