Ronaldo has scored just eight goals in 17 La Liga outings this term - well below his usual average - but has been impressive as ever on the continent, finding the net nine times in six group-stage outings.
With seven days to go until two of the world's most-expensive players face off, Mbappe claims that there is little to suggest the five-time Ballon d'Or winner is losing his goalscoring prowess.
"Ronaldo is not on the decline. If he is on the decline, there are many more players on the decline then," he told reporters after PSG's 4-1 cup win against Sochaux on Tuesday night.
"He is a great player. But we hope he will remain quiet against us. We will prepare for the game by paying attention to all their strengths. And he is one of their strengths in the team."
Madrid have won the Champions League three times in the last four years, including back-to-back in 2016 and 2017.