Mueller is being discredited because he used Hillary's little black book to choose his staff. Almost all the people on his staff worked on Hillary's campaign, were Hillary donors, or were attorneys for Hillary or entities related to Hillary. Mueller himself is BFF with Comey, who was the person who leaked confidential information to the press with the deliberate specific intent of creating a special investigation.
More than just that, his staff is bloated beyond belief. At last check, his staff was up to 275 members and it is still growing by chain nomination. Mueller brings a guy on, and then that guy brings on all his friends and family, and then the friends and family members bring their friends and family into the investigation, and it never stops. Unless Mueller is shut down, soon his staff may top the staff of the EPA.
He is not 'conducting an investigation', he is creating a brand new Hillary supporting/Hillary led government administration inside the Trump administration.
Mueller is not being criticized for what he is 'finding'. Because he hasn't found anything. He is being criticized because of his openly pro-Hillary, pro-Comey, stance, and his cadre of Hillary cheerleaders that he brought with him.