Mbappe has flourished at PSG since arriving from Monaco, helping the French giant to back-to-back Ligue 1 titles.
The 20-year-old FIFA World Cup winner with France 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 said: "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."
Mbappe scored as PSG opened its Ligue 1 title defence with a 3-0 win at home to Nimes last week.
PSG is away to Rennes on Monday (AEST) amid uncertainty over the future of Neymar, who continues to be linked to LaLiga champion Barcelona and Real Madrid.