Your first post, welcome to FreeCAD.
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Is the object a solid?
Did you modelled in FreeCAD?
If the original is a mesh like STL then you will never get nice curves, they will always be polygonal.
If it is a solid made by someone else, perhaps a STEP file, it is possible that some one else imported a Mesh and then converted it to a solid, which would still leave all the polygonal curve approximations.