I mean, he still is. Fangraphs still had him as the 3rd overall prospect in all of baseball when they last updated their rankings. He's a great prospect. A 142 wrc+ at AA as a 21 year old is nothing to sneeze at.Shows you how good our system is and has been where Mayer is almost a forgotton guy now. He would be a top prospect in many teams lists.
He's just had more injuries and hasn't touched AAA yet and is more obviously blocked at SS, so it's hard to see him making an immediate impact next year.