The best way that I have found (up to now) to play against MK would be playing against the MK user defensively and on the ground mostly. That way you can avoid MK out prioritizing you in the air (cause your not playing in the air obviously) and you have a more safe approach while fighting him. What I mean by defensively is stay on the ground, and shield his attacks, grab out of a shield, and dodge his grabs (you can do other things but start with that). Also, hit MK with smash attacks.
Only thing though watch his Dsmash play, space a bit ahead of him so that after the first blow you can come in for a smash attack or grab when he is making the second hit of the Dsmash. Also Fireballs are your friend, even though they don't rack up much percentiles at least it is something and that little bit of % can be enough to score a KO. Try to mix up your ground game as well so that you may be a bit more unpredictable.