Mbappe has flourished at PSG since arriving from Monaco, helping the French giants to back-to-back Ligue 1 titles.
The 20-year-old World Cup winner scored a league-high 33 goals last season, and 39 across all competitions.
Diallo – who left Borussia Dortmund for PSG in the off-season – knows Mbappe from the pair's time together at Monaco and the centre-back told reporters: "He grew up, he gained a bit of weight.
"But he is the same. Of course, he is more mature. Now, we can talk about more things with him.
"When I was in Monaco, he was very, very young. But he is still the same.
"In the game, he is becoming a real killer. He didn't have those qualities when he left Monaco, even if he scored many goals for Monaco. Now, he is very accurate."
PSG are away to Rennes on Sunday amid uncertainty over the future of Neymar, who continues to be linked to LaLiga champions Barcelona and Real Madrid.