No, it mutates because because it comes after a short-form verb (Gwariodd e) i.e. it's the object. It's the same mutation you get in:
Bwytodd e ddeg afal "He ate ten apples"
Helpodd hi ddeg o bobl "She helped ten people"
Gwelodd Mari ddeg ci "Mari saw ten dogs"
Each time the deg mutates after the short-form construction.