No, platies are not aggressive. They are peaceful and rarely fight with their tank mates, but sometimes male platies bully each other.
However, in certain situations, male platies may behave aggressively towards other platies.
First, as livebearer fish, male platies always want to mate, so they may harass female platies frequently. This isn’t exactly aggression, but it can stress out the females, so you should keep a 2:1 ratio of female to male platies.
Male platies may also show aggression towards other males to assert dominance, especially if the tank is overcrowded. They may sometimes fight over food and mating privileges as well.
To prevent bullying from male platies in the tank, make sure the fish have enough space to be away from each other and plenty of food. Keep a ratio of two females for every male, and avoid mixing different types of platies together.