StarterCharacter isn’t used for this. The purpose of StarterCharacter is primarily to change how the rig is built, whether you need to change the limbs of the player or have them spawn with other things on them (extra parts, values, all that).
If you want to override clothing, there is an option in Game Settings which allows you to apply default clothing for all players who enter. If you need more than one outfit applied to a player, then you can explicitly change their outfits by using a HumanoidDescription or directly changing (or adding if there is none) their Shirt and Pants instances.