Mook said on "Squawk Box" he found it "particularly outrageous" that Trump called himself "smart" for taking a $916 million tax loss in 1995 and possibly not paying income tax for nearly two decades.
"It just makes no sense for the next leader of our country to be running around saying he's 'smart' because he didn't pay taxes," Mook said. "What does that say about the rest of us who work really hard and pay our taxes."
Mook said he's interested in hearing what Trump running mate Indiana Gov. Mike Pence has to say about taxes in the only vice-presidential debate Tuesday night against Clinton's running mate Virginia Sen. Tim Kaine.
Trump campaign manager Kellyanne Conway, appearing earlier on "Squawk Box" Tuesday morning, said the GOP nominee's actions were legal. "It's not a loophole. It's a provision in the tax code."
In addition to attacking Trump on his taxes, Clinton took aim at the Wells Fargo phony account scandal at an Ohio rally on Monday, as the Democratic nominee outlined plans to crack down on corporate excesses.
JPMorgan Chase Chairman and CEO Jamie Dimon came to the defense of his industry, while calling out Clinton for what he said was "unfair" comments in which she used blanket statements to condemn all bankers.
In reaction to Dimon's rebuttal, Mook said: "Some of the practices [at Wells Fargo] that we've seen are simply unacceptable. I think Secretary Clinton was simply trying to be clear that she will be tough if there are abuses in the system."