Yes, the silkscreen and copper layers define presence of material, i.e. any figure (graphics) in these layers mean that there is material in the corresponding manufactured board layer. Mask is negative: graphics means absence of material, i.e the mask substance which otherwise covers the board doesn’t exist where there is graphics in the design layer, either PCB Editor or the gerber file.
No, but you should consider readability. It depends on the needs. It’s hardly a good idea to put green mask text – either positive or negative – directly on the green board material if someone needs to read it easily. If you put mask text – either positive or negative – on copper, the result depends partly on the copper finishing if it’s applied to all bare copper.