The illustration above is from Dan Jones’s book Summer of Blood: the Peasants’ Revolt of 1381. Part of the caption is “Queen Joan, Richard II’s mother, pleading with the rebels as the Savoy burned”. Elsewhere in the same book, Joan is referred to as the queen mother.
According to Merriam Webster, the first known use of ‘queen mother’ is 1560, and it is defined as “a queen dowager who is mother of the reigning sovereign”. Joan, of course, was never a queen. She was married to Edward, Prince of Wales (the Black Prince) who died before he ascended the throne. So Joan was only Princess of Wales. This applied even when her son became Richard II. Being the mother of the king didn’t make her a queen (ask Margaret Beaufort!) so Dan Jones’s caption is incorrect, and should read “Joan, Princess of Wales, Richard II’s mother, pleading with the rebels as the Savoy burned”.