In fact, Maxis encourages and supports the modding community! You can read more about that on The Sims 4 Mods and Game Updates FAQ page.
Custom content is created and tested by users-it's a big part of the Sims community. This content often includes clothes for your Sims, traits, aspirations, and much more.
Custom content, or 'Mods,' is additional assets and behaviors created by other players for the purpose of enriching the game beyond the base game that Maxis has published.
