I could be wrong, but I do not think tiles in a Tilemap have names, at least not internally. I believe they are stored as integers internally, which is why get_cell returns an int rather than a string. In the tilemap editor though, in which case, it should just be a matter of opening the scene you used to create the tilemap resource, changing the names of the nodes, and then exporting as a tilemap library/resource in the Godot editor like normal.