Try replacing the material on the outfit to one with no texture (make sure the material is completely empty) , If it looks flat then; issue lies in the texture of the outfit, the boobs are probably painted on.
You can take the texture into a paint software and paint the boobs out. Or just don't have a texture at all.

If the outfit still has boobs with a material with no texture. You can use mochifitter mesh editor and edit the mesh to be flatter.

if all else fails You can go into the armature of the outfit and manually move the chest/breast bone to be more aligned with
your models chest. You will have to unpack the prefab to do this though and so it not recommended