It's not true.
I heard a story that Marissa Tomei was never meant to win her oscar, but the wrong name was read out and they went along with it anyway.
Sounds like a case of sour grapes from someone who felt someone else should have won.
I absolutely adore Marisa Tomei in My Cousin Vinny. She's fantastic and totally steals the film. It was a breath of fresh air that the academy recognised a wonderful comedic role