I would disagree. The Mandalorians aren't even a nation - they are a nomadic, barbaric tribe who's culture revolves around war. I can't see them keeping hold of the Core Worlds which would need government and policing - I mean do you think the Mandalorians would be equipped to deal with the Cold War uprising on Coreilla? Even the Empire couldn't handle that. I believe as the war begins to peter out, and the Mandalorians begin encroaching on the core worlds, they would begin to fall apart, fighting each other and fighting insurgencies. And the Republic would eal the killing blow. Ultimately they could not govern a galaxy.
I have no doubt that the Mandos could have conquered the Core worlds, but it's unlikely that they could have held on to them.