I'll withhold judgement on Z for a few years. I remember watching Mac get started - he was such a disgusting brat - MUCH worse than Z - it was intolerable, no matter how well he played. Then there were other nightmares, like Connors. Remember Muhammad Ali when he was getting started? Holy smokes.
And it's a virtual certainty that a young kid like this, barely old enough to shave, may have difficulty adjusting to relatively sudden worldwide fame and tens of millions of dollars. I mean, it's not like he stepped onto the city park court next door to hit some balls. These are not "celebrity" personalities to begin with; they didn't grow up seeking the limelight. They're just ordinary guys like you and me, who suddenly became international rock stars just because they're good tennis players.
So.... yeah, he's annoying, but I find a lot of 21 year olds annoying. Let's see where he goes from here.