The Premier League giants moved into the Carabao Cup quarter-finals after the tight win at Stamford Bridge on Wednesday.
Tomori and Richard Keogh scored own goals for Derby, who fought back on both occasions through Jack Marriott and Martyn Waghorn, before Cesc Fabregas' 41st-minute strike proved to be the winner.
Mount and Tomori are both on loan from Chelsea and Zola enjoyed the duo's performances.
"He [Tomori] has been unlucky with the own goal, but the performance he and Keogh put on afterwards was excellent," the Chelsea assistant told a news conference.
"It happens. If he hadn't touched that ball, [Alvaro] Morata would have scored a goal. He played well.
"Mason Mount played excellent. It looks like they're in good hands. They're doing very well for the team. The coaching they're getting is excellent."
Zola was unhappy with the way Chelsea finished the encounter, while he praised Frank Lampard's Derby.
"More than a little bit [shaky], to be honest. Especially towards the end," he said.
"A lot is down to them. They really played well and surprised us, played with a lot of quality.
"A lot was down to us. The passing wasn't as sharp as it normally is, and the last 10-15 minutes we lost control. The distances were too far apart, and when you give space to the players Derby have got, it becomes difficult.
"It was positive in some ways, but others didn't work properly."