Was Bernardo Mercado an underachiever?

This man was 6'6" and could punch with the best of them and won 20 fights in a row while compiling a record of 39 fights with 5 losses, no draws which included a remarkable 29 knockout victories and a few of them came in the first round. His victims included 27 year old Trevor Berbick - who he knocked out in 1 round and whereas it took prime Tyson 2 rounds to beat 34 year old Berbick - and Earnie Shavers, who he also knocked out, albeit Shavers was a little past his best at the time but Earnie did go on to beat Bugner in 2 rounds not long after and knocked out Norton in one round and floored Holmes the year before he fought Mercado. After his win over Shavers, Bernardo was ranked the number 1 contender for Larry Holmes' WBC title before Leon Spinks stopped him in 9 rounds hence Spinks going on to fight Holmes and losing in 3 rounds.