Is it or is it not legal to remove importation marks that are stamped on all firearms imported into the United States?

I cannot find any reference to this particular subject.

All I can find is the GCA reference that requires importer marks on all firearms that are to be imported into this country (no reference to what happens afterwards), and a reference to the illegality of removing serial numbers from firearms.

But I can't find anything referring to removal of  importer marks, specifically.

On CZ pistols, the importer mark is laser-engraved onto the right side of the dust cover of the frame. CZ pistols come with a "polycoat" (spray and bake) finish (as well as others), and the importer mark is 'engraved' on top of and through this paint finish. If one were to strip the gun for refinishing, the importer's mark would most likely be obliterated beyond recognition.

If it was illegal to remove, wouldn't they have made it a bit more permanent than laser-drawing it on top of a coat of paint?

It is not unlawful to remove an importer's identification mark. See my detailed reply in the General Discussion.