What is the reason that Red Dragon is testing supersonic retropropulsion on a Mars mission. Is the Earth's upper atmosphere too different to that of Mars to validate this method as a viable way of landing on Mars?
They have been testing supersonic retropropulsion on Earth: the Falcon 9 landings do their reentry burn in circumstances not too different from Mars' atmosphere.
I've seen no announcement of a Dragon supersonic retropropulsion test on Earth, but that doesn't mean there won't be such a test. SpaceX does a lot without publicly announcing it.
The Martian atmosphere is very much thinner than Earths. So SRL on Mars needs to happen in the equivalent of the density of Mars's atmosphere.
Thus when the Falcon 9 first stage is coming back for a landing on the ASDS ship (No longer a barge! Don't be a barge-ist on this site!) or back at LZ-1, they are collecting reentry data to share with NASA as it passes through similar densities.
There was a great presentation by Larry Lemke, at SETI talks where he talks about EDL (Entry, Descent, Landing) on Mars, and that there is a 'sporty' way to do it with a lifting body capsule, where you would actually fly down closer to the surface, into the densest part of the atmosphere, and then fly up, to stretch out how long you have to decelerate via aero breaking.