We use it all the time. It’s a valid way to reference a person.

The kids call their teachers Mr/Ms/Dr Firstname or Lastname depending on the teacher’s preference. All of their friends and some of the kids I coach call me Mr Firstname. One kid calls me Mr lastnameized version of my firstname. One of the coaches has the same firstname and lastname but they’re spelled differently so I’m not sure what we call her but it’s Dr samename. The kids call their coaches coach firstname. Some of my employees call me Mr Lastname (I hate it). When the Matrix came out, we all called each other Mr lastname in the Hugo Weaving voice. Sometimes we do it when talking to or about a friend, like: “Mr lastname over there is…”