Should you use a thicker tire on a BMX if your a bigger or heavier person?

Depending on how heavy you are. You might either get away by running higher pressures, or you might need wider tires, or you might need to use BOTH wider tires AND higher pressure. It all depends on how much clearance you have between rim and the ground when you're on the bike. Higher pressure/wider tire will increase that distance, and that's what you need to avoid damage to tire and rim on landing.