I suspect a lot of people who are having children now, or will be in the next few years have mothers named
Mary, so I suspect it's not quite in the trendy-Grandmother names stage yet. However it's a classic that's never really been out of style in 500 years.
I am a 20-something year old
Mary so I tend to notice other young
Mary's, it's not as uncommon as people seem to think.
I find a lot of authors seem to tend towards names that are currently popular rather than really common names. Except
Kate, I swear every second character is called
Kate.