From Longman Dictionary of Contemporary Englishham it upham it upinformalAPACTOR/ACTRESS to perform with too much false emotion when acting → ham
Examples from the Corpus
ham it up• For all the kids care he could be Goofy, hamming it up for Mickey Mouse.• Every year Dad puts on his Santa suit and hams it up for the kids.• Overemphasis, hamming it up, leads to the exaggerations of satire, cartooning, melodrama and farce. 