To be clear, the claim is that Mitch Marsh was (as of 8th March 2017 - and continuing until the 2017 Boxing Day Test Match) the worst #6 batsman in the history of Test Match cricket. Not just the worst Australian #6, but the worst #6 batsman for any nation in the world.Theres been a number of Test players worse than Mitch Marsh.
Not many have had the longevity in the system mind you
The qualification here is that the player must have batted at #6 in at least 20 Test matches.
Mitch Marsh held the title for approximately 9 months, in 2017. He no longer holds the title, and currently sits 12th on the WOAT #6 batsmen list.
The claim is not subjective. It is based on his batting average, over a prolonged period of time (i.e. min 20 Tests). It is an objective fact. He was the WOAT, just as Lyon is the GOAT.