Pierrot and Pierrette were characters in pantomime, a kind of comic fantasy show that was popular in England, especially during the Christmas season. I think it went out of style sometime between the world wars. The other main characters I know of were Harlequin and Columbine. They all had distinctive costumes--I think Pierrot's was black and white, with black and white makeup to make him look like he was crying.

Pantomime was based on the Italian commedia dell'arte, via French. I'll bet anyone who knew that fact knew I wouldn't be able to resist passing it on.