Yes, but just because one's a fine duelist doesn't mean they could stand up in, say, a bare-knuckles brawl, or a fleet-on-fleet battle. Mandalore was a brilliant tactician in addition to a beast in a brawl - and Exar always reliant on his lightsaber in a fight, and he was a fighter, not tactician.
Mandalore the Indomitable had all of the advantages in his battle with Ulic Qel-Droma, and that didn't work out him. Ulic pretty much destroyed him. Not only that, but Kun is one of the most powerful Sith of ALL time. He's up there with Vader and Dooku, if not better than them. Kun could easily best a large group of mandalorians single-handedly. Mandalore is no match for him. He was so powerful that the Jedi Order was forcd to lock him on Yavin 4. They were scared to face him in battle. He bested his master in a duel when he was just a Padawan. He killed Odan-Urr with relative ease. Mandalore, and any other Mandalorian, is nothing compared to him.