I sense some confusion in your question.
The mantle lays deep below the crust, therefore if a body goes into the mantle won't be visible on the surface.
But let's proceed with order.
As you correctly guess, an impact with a planetary body will bear large destruction. The energy at stake is so large that the entire planet would be melted. As a consequence of the melting, the liquids would stratify according to their density: denser materials would go down, lighter materials would float.
Therefore, if the colliding body was made by a high density material, like iron, it would sink, going into the mantle or below, into the core.
Moreover, since the whole planet would be liquid, there is no chance that surface features higher than the average level would be permanent. For that to happen, you need a solid crust. But once that is present, stratification has already worked.