I'm from West Glamorgan. I can think of least 4 males named Ceri in my school years, and no female ones. You can look up Ceri Sweeney who played rugby for Wales. I have only come across a female Ceri in recent years and then in print not person. Maybe the tendency for women to adopt male names over time (e.g. Hilary, Beverley, Chris, Charlie) is affecting Welsh names too!
In Welsh "mam" IS the word for mother so it is not slang. Presumably as (standard, ruling class, south-east England) English entered Wales many Welsh people kept using mam as the word/name for their mother and it became an element of Welsh dialects in English. Incomers, along with media and school influences, are apparently reducing this Welsh dialect.