Williams surprised team management, by taping over BNZ logos on his jersey earlier this month, citing Muslim religious beliefs that don't support banks, alcohol brands or gambling sponsors.
Instead, the All Blacks midfielder will carry the logo for Plunket, a charity designated as a substitute by BNZ.
Mexted told Radio Sport's Martin Devlin that Williams' stand was contrary to everything that constitutes a healthy team culture, which he believes is lacking at the Blues.
"He's contrary to everthing that's good about a team ethos," said Mexted. "For years, everywhere in the world, people have been saying there is no 'I' in team - teams are all about buying into whatever the team goals are.
"Think about this sponsor, BNZ ... if there were three or four players like Sonny Bill that refused to have their logo exposed, what happens next year?
"Do they pay the same money for the sponsorship? No, they don't, because a quarter of the team aren't wearing the logo."
The former NZ No 8 says the Blues have some outstanding players, but they're not a team and pointed to the resolve shown by the Hurricanes in their Super Rugby clash on Saturday night as symptomatic of a strong team culture.
"If you asked me why can't the Blues win ... well, I don't think the team is good enough.
"The players are, but when it comes down to fighting out a victory in the last 10-15 minutes of a game, you saw the resolve in the eyes of the Hurricanes under pressure and they came through.
"The Blues aren't going to do that until they have that same team concept and [Sonny Bill] doesn't add to it, that's for sure."