It's not uncommon; There's a title for it - "Brand ambassador" - Though it's often the case that your official job title is something different while you act in that role. I doubt people in that position have illusions about what their role is, and in fact most are probably offered the choice of how much they lean into the branding and how much "Real Work" they do, with expectations set around "You are famous/known for X, we want X people to like us, go off and talk about X to cast us in positive light".

Pretty comparable is "Developer Advocate". The job is to grease the wheels between people using your services and the developers developing them. In that role, they playact as both customer and provider, both to developers and consumers. An awful lot of Developer Advocates are mediocre at both, but you don't need to be good; In fact, in some ways being mediocre is a plus, because the point is to represent each side to the other.

Whereas, the people who have done great things and spend their time on brand instead of "Doing X"; Often they are coasting on reputation, but it's a form of retirement for them. They still like X. They still want to do X, but as a hobby, and getting paid to talk about X and get others excited about X is like having your cake and eating it.

Brand ambassadors are a different thing. Thats like when a famous rapper is the official fan of an NBA club. Employees hired on to positions with technical contributions (developer advocate) which is just a cover for some kind of image building in the market.. dunno