I had the impression
mummer was reasonably well-known, though I think I'd only ever run into the word in books. Mummers featured in one of the Ngaio Marsh mystery novels, and I think they were in a Thomas Hardy novel I read.
However, it seems from the comments here that quite a few players don't know the word. It does appear from time to time in news media, mainly in connection with specific locations, such as Philadelphia, as mentioned by RM and birdy, and in various places in England, Ireland, Canada, etc.
An article on the Canadian CBC website last year was headed "The dark side of mummering: What you may not know about a beloved tradition" and dealt with mummers in Newfoundland and Labrador.
Probably the use of the word
mummer is not widespread enough for it to remain as common. The same applies to
mummery, which has also been a common word up till now. Both will be classed as rare in future.