You have the right idea but the wrong time direction. The evolution is "Mador(e) -> Mather". A common name in Austria is "Mader" and I would guess that it has the same origin. The root is "mad" which is Keltic for "good". The "or(e)" part comes from another Keltic root "ar" of which I have forgotten the meaning. There is a German name "Wohlgenannt". I thought this odd until or course I realized that it was almost the translation of mine. It is debatable whether these names designate someone who does good deeds (we would hope) or rather someone who likes good things. King Arthur's knight "Sir Mador(e) of the Gate" did not excel in either quality. The earliest occurrence I know of the name is in the anonymous novel "Sir Gwain(e) and the Green(e) Knight". The longest description of "Sir Mador(e)" is in the novel "Le Morte d'Arthur" by Thomas Mallory. When the Saxons invaded England some of the Britons fled across the Channel to Brittany and it is from there I would guess a fair number of them crossed the Atlantic eventually to Maine and elsewhere, perhaps even having been once again chased by the Saxons (now calling themselves English) from Acadia.