Not really. It was a taunt she said to annoy him. Regardless of how knowledgeable she is about such matters, anything said in mockery should not be treated as a factual statement.
For all we know perhaps she really thought that Kun would have wiped the floor with Vader and the Emperor would have respected him. Would you expect her to actually tell him that? Of course not. People don't always say what they really think.
I usually take statements at face-value, not to mention Mara herself knows when she encounters a powerful enemy she usually tends not to provoke them. Anyways there's no such quote that has Kun actually say he considers Luke's skills or power quaint or whatever, in fact from the Jedi Academy Sourcebook it states numerous times that Kun would rather not face Luke directly because he's too powerful, which is why he tried to possess his students and then try and take over Luke because of his power.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk