From the Star-K: Shatnez in Oven Mitts

In Important Alerts, Newsletter Items by kosherqu

Cuisinart brand oven mitts, product of China, with label stating “100% Polyester Filling” contain a shatnez filling.  They are not pure polyester as they contain wool and linen.

In general, oven mitt fillings should be checked for shatnez, especially if they are made in China.  Silicone mitts without a filling do not require checking.