The problem with the question of "is he ready" is defining the term ready. Has he been around long enough for the good pitchers and scouts to find and exploit his weaknesses? Has he been able to adjust to those exploitations? Is he physically strong enough to withstand the rigors of a full season.
The situation we are currently in should have taught us that six years of MLB control go swiftly. If we bring a player up that is going to struggle in the majors, even if he learns as he goes as well or better than he could have in the minors, we might have wasted two or more if his controlled years. Much better to do his learning, as well as his physical maturation in the minors. We had lost years with Happ. Lost years with Javy. And lost years with Schwarber, all of which has come back to bite us now. It was perhaps understandable in 2016, when we were going for it all in a year we had a reasonable chance to make it. But I highly doubt that Hoerner's presence, no matter how well he does, is likely to bring us to the World series.
I agree with Deeg that spring training stats mean very little, especially this early, when more than half of the pitchers faced will probably be in the minors or out of baseball in three weeks. At the same time, it seems silly to say that thus far, Hoerner has stunk. A .247 major league BA for the first 186 at bats is hardly stellar, but is probably as good or better than most rookies his age, and came with excellent defense. Hoerner contributed in his first year or so, but I doubt that his contribution will be very much above average this year, and will end with another wasted year of control.
He may be ready to play at a reasonable level this year, but I doubt highly that he will be able to excel, as he might do with another year's experience and maturation. I hope they do not start him with the big league team this April.
All this comes with the caveat that those who make the decision will have so much more information at their disposal to make that decision than we have that is doesn't make a lot of sense for any of us to second guess that decision.