Mace wouldn't really be a fallible source if he's saying that someone is better than him. I think he would be the one to know. Moreso than if she said it or if even Yoda said it. He knows his skill and knows that her bladework is better. Now, does that make her a more complete and more dangerous combatant? Not necessarily. Just like Anoon's blade work is second to none, but that doesn't mean that Yoda would fall to Anoon. Same thing here.