Does mindfulness end kamma ? is that a good thing ?
What is wholesome is what you perceive positively at the time the result manifests. Since all worldly experiences are unsatisfactory, experiencing Nirvana is wholesome. Anything that reduces your stress and suffering is a good thing.

When you are practice Right Mindfulness, you do not create new Karma then your old Karmas start surfacing which reduce your old stock of karma and newly created karma. Development of the path factors is good Karma. Path factors are also part of the Mental Factors (cetasika).