Kevin Durant Fined $25k for Cussing Out Mavericks Fans Over 'Cupcake' Insults

[[tmz:video id="0_yluu1qzp"]] 11/20 -- The NBA has fined Kevin Durant $25,000 for "directing inappropriate language toward a fan" 2:38 PM PT -- Another witness tells TMZ Sports ... the hecklers had been bombarding KD with the "cupcake" insults to…