Usually this means that the model was created using a technique called subdivision surfaces. Subdivision lets the artist model the asset with low resolution geometry and then 'subdivide' it to make it smooth.
Stager does not support subdivision surfaces natively. Whatever tool the model is created in and using subdivision surface must 'bake' that information down into geometry at export. Unfortunately this is not a bug or something we can fix. You can attempt to open the model in a tool that does support subdivision surfaces and then bake out a new version of the asset that is smoothed.