It's the Simmers' choice to use / install custom content or not. While modding and sharing are common practices in the Simmers world, you should always take into account the potential risks that may derive from importing custom content in your game.
Several things, from recolors to new meshes for hair, clothes, etc., can be added to the game. There are already several tools that give us the ability to create new stuff for The Sims 4.
