If the question is, would the Mayor have remained the giant snake/slug/crawly demon thing after the Ascension ritual was over and his ultimate plans had been executed, I seriously doubt it. He mentioned once, I think in the recording that he left for Faith in Season 4, that he expected to be his normal, human self once more. The beast was most likely just a temporary manifestation that would have given him the power to carry out his plans. That, or a form that he could adapt at will, something like the way that Illyria would later be able to shift back and forth between blue haired hell god and normal, bright eyed Fred, but with the added size change capability between shapes.