Aguero scored a brilliant opening goal and then teed up Kelechi Iheanacho to double the advantage at Carrow Road before Kevin De Bruyne added a third late on
The Argentinian's season so far has been interrupted by niggling injuries, but he made his fourth consecutive start and was back on form in a superb display.
His opening goal came after a trademark weaving run, in which he ghosted past two Norwich defenders before evading Ryan Bennett's last-ditch challenge and poking home.
"When Kun continues playing games and playing more minutes, his performance always improves after he comes back from a long injury," Pellegrini said.
"Maybe in the first game back he didn't play so well
Last game against Everton he made a lot of better moments and today he continued making those moments and scored a beautiful goal.
"We know he is a top player and when he continues playing he will arrive at his normal performance."
Having cruised into the fourth round, City will now turn their attention back to the Premier League
They trail leaders Arsenal by three points.
Aguero's return to fitness and form is a timely boost for Pellegrini, who admits it is key to his side's hopes of silverware that the striker remains injury-free.
"When you have players as important as Sergio it is very important for their team-mates, for the manager, for the fans to see them playing every game because he is a player that makes a difference," Pellegrini said.
"We are in the semi-finals of the Capital One Cup, three points off the lead in the league, we have qualified in the Champions League and we have started on the right foot in the FA cup.
"I am very happy about Sergio and I am very happy with the whole squad."
City's other strikers have struggled to fill the Argentinian's boots when called upon this season, but Pellegrini insists he is not looking to sign any new players in the transfer window.
"For the moment I am very happy with the squad," Pellegrini said.
"We always have the option, it is not closed, but really we are not thinking about anyone at this point.
"Players out, the same thing
We will keep the same squad."
Canaries boss Alex Neil said he was "disappointed" with his side's performance but made clear after the match that his priority lies with the league.
"If I'm being honest, for us, we'll forget this game as of just now because our bread and butter is the league," Neil said.
"We made seven changes today so three quarters of the team were changed and some of those players only played out of necessity rather than anything else.
"It's a game we wanted to do well in and we didn't perform as well as we should have done.
"But as I say it's not something we're going to beat ourselves too much up about."
Source : PA