"Mẹ yêu" is phares used by a son/ daughter to call his/her mommy or talk about his/ her mother or mother - in - law ( in some special case such as she/ he love her as much as her/his real mother)
In fact, It's not a popular way to talk to Vietnamese mother, you can use it if you want to show your feeling or ask something..... Most of user is female ^)^ ....... Male don't like that way ^)^
you can use it for boy " Em /con" or " chị/ anh" + yêu . With older person, you need to put " Kính / thân" before "yêu" such as" Cô/chú/ông/bà" + "kính" + "yêu"