Kostümschinken means literally "costume ham", but no, it does not refer to an actor prone to over-act, but to a historical movie distinguished more for the lavishness of its costumes than the quality of the dialogue or acting--think Victor Mature (Samson and Delilah), think Charlton Heston (The Ten Commandments), think Yul Brynner and Gina Lollobrigida (Solomon and Sheba). More recently,
Marie Antoinette has been considered by some as a modern version of the genre. More in my first comment...