Aguero, 26, scored his third hat-trick of the season, and took his tally for the campaign to a career-best 31 goals in all competitions. The Argentine moved closer to winning the Premier League Golden Boot award, with his haul of 25 goals five more than the 20 of Tottenham's Harry Kane, with just two matches remaining in the season.
"When I see Kun playing the way he played today I'm happy how he is improving," Pellegrini told reporters after City demolished QPR 6-0. "It is not the same Aguero that played one year ago.
"He understands he must work harder, he receives a lot more ball. He is working harder during the week. In the way he continues improving, he will continue improving in the future.
"He is just 26. He must be a very important player in the future of this club. He was already but I think he must continue improving in the way he plays."
Pellegrini was delighted with the fact that his side registered their largest win of the season, but added that keeping a clean sheet made their victory that much sweeter.
"We never change that philosophy of football," he said. "It is important also to keep a clean sheet, to be a balanced team and not concede easy goals.
"That is why I am happy not only for the six goals, but because we were a balanced team and played with a lot of spirit to win against a team that took a lot of risks because they needed to win."