Against ness you are going to want to focus on grabbing him and edgeguarding him.
Playing onstage works, but only if he is on the ground or above you. The battle pretty much becomes you picking your moves carefully. D-tilt is good if he is on the ground (Dash grab at 0 after d-tilt is almost guaranteed, and it is guaranteed if they trip). His **** does a lot of damage really quickly (PK Fire bat does like...45%). Just be patient enough to not get hit by the PKF stuff and you should be good.
If he is in the air facing you, his fair will beat anything you have. If he is facing away, space your uairs so that you can get a uair nair without running into one of his hitboxes. Don't try to punish fair unless he literally falls right on top of you, in which case you can shield grab or nair OOS. He is very limited if he is anywhere above you in the air unless you are in the fair bubble range. Abuse that with your fair and uair, and sometimes even dair.
You will learn through playing it what moves can mess with what in certain situations, so I can't really teach that. You must learn that for yourself (it relates to the player habits too much for me to say X always works).
The key to this MU is edgeguarding. Honestly, with how much better I've gotten at it, it seems like +1 Pikachu. You can stop his recovery with a plethora of things in several different situations:
If he is below the stage, still has his DJ, and could DJ to where he could land on stage (height wise), you should wait for the DJ and hold out jab. If you notice that he starts to up-b, just run off and nair immediately. Get ready to tech because the tail of PKT
might hit you, but that comes down to timing.
If he is out there with nothing and is forced to up-b, jump out there facing the stage (Back to ness). Uair footstool the **** out of him when he starts to PKT. It is stupidly broken, and depending on where he is you can do it once, QA to the ledge, and do it again.
If he recovers high, just respect it and try to hit him back off when he comes down with fair/uair as I said before. Nair is hard to land on him in the air, but you can do it with practice.
If he recovers low but far enough out that he could DJ fair your jab attempt, you could T-jolt him. T-jolt will beat everything but nair (which you could then punish) and magnet. If he magnets, he is lower and now you can edgeguard him with edge grabbing and uair footstool. If you wanna be ballsy, you could go for a QA down-off and then up-on stage. It could interrupt him, I've done it a few times vs Shaky and killed him for it.
Example videos~
me vs coney
http://www.youtube.com/watch?v=QsEXXo28KLw
me vs shaky (Old, you won't see most of the edgeugarding I discussed)
http://www.youtube.com/watch?v=lq9fyy0TP4Q
http://www.youtube.com/watch?v=PIwzRd-V4AY
http://www.youtube.com/watch?v=HdXBxpsQxDA