grounded UP-b from MK beats all out approaches, so we MUST camp in this MU with lag-cancelled retreating pk fires, pivot pk fires, pivot grabs, Dair dancing, retreating fairs
if you really want to approach, SH double aerial shuffle or SH dair (probably worst choices), SH airdodge (probably best choice, to bait a shuttle loop or dsmash/tilt then grab, nair or jab combo), or a dash grab.
however all these are easily read and punishable, only do these if you've read your opponent, as a shield grab will negate all of these approaches. lag cancelled pk fire can be punished with uair i think or shuttle loop, not too sure, but i know its risky. I do not know about PK jump in this MU.
do not chase mk off stage, just get good at PKT and punish him several times with that.
Punish a grounded Shuttle loop into you shield with Uair i think.
do not Dair into MK, very easily punishable (either by utilt, or shield grab, or Shuttle loop)
punish barrel roll apporaches with the "Dair dance" (lag cancelled dair's at a safe distance, term coined by Eagle)
punish dash attacks/grabs with pivot grabs. save bgrab until like 115%+ (depending on your position)
punish dair camps (must be will timed) with aerials. f/b/nair are used depending on Mk's position above you.
if youre off stage, jump and airdodge back on is probably the safest thing, and DI his dair punish to the other side, or get grabbed. sometimes the MK will overcommit to the gimp and youll land back on stage fine. sometimes you can bait them into the tail of your PKT and kill them if they are not well versed in the MU. remember to stall your PKT2 if hes edge-hogging, but make sure you can make it back to the ledge or back on stage. if all hope is lost, send the pkt at him for some dmg lol.
if youre up high, dont fall into an MK, aim for the ledge or somewhere safe. he can just shield grab, dash grab, dair or shuttle loop a landing. beware of dtilts and ftilts if you aim for the ledge, try fast falling to screw up his timing. (i sometimes PKT2 myself into the ground to force that lying down animation, or PSI magnet stall)
If the MK is uair planking at the ledge, here is a video about what to do (not sure if it applies to ness, as it requires an aerial that hits at or before 13 frames, thus we cannot dair, but here you go):
http://www.youtube.com/watch?v=2Joyf8KKdg4&feature=bf_next&list=SPF52238E65D6ED18D
i think the best bet vs mk is getting a stock lead and camping for the game lol.
note im not that good, but this is what ive gotten from playing my friend A LOT
hope this helps :D